WMG: Kantai Collection

The ships represent not the grudges of the shipgirls but their ambitions
This would at least apply to the Abyssals which have clear ties to active shipgirls. In classic Japanese Spirit fashion the 'souls' of the defeated IJN ships were literally hell-bent on overcoming those that defeated them and ended up following Imperial Japan's method of taking what they deemed advantageous from the superior side. This would reconcile the idea of them being ex-IJN (and Third Reich) ships with that of them being Allied ships at the same time - they're Axis ships supernaturally 'modernized' with Allied equipment.

The game, as it were, takes place in a proxy afterlife

This was brought on recently by the realization that the (as of this writing) Fall 2013 Event is based on the freaking Naval Battle of Guadalcanal, down to the bombardment of Henderson Field for the final phase of the event. Heck, the third phase (E-3) is even titled Ironbottom Sound, the area between Savo Island and Guadalcanal where many ships the kanmusus are based on met their end.

This made me think that what we're really seeing is the spirits of the IJN ships (or maybe the resident kami that Japanese believe to reside in everything) are unknowingly reliving the encounters that got their "bodies" sunk in the real world. Given how many of these ships met violent ends (it was war after all), their spirits are fighting these battles again and again until they let go of whatever grudges they had upon sinking, so that they could move on to the next phase.

Which leads to the next whacked out theory...

The Abyssal Fleet the kantai girls are battling aren't simply the "grudges of sunk ships" as implied by the 4koma...

They're the grudges of the Allied ships sunk by the IJN during the war. This theory was brought about by the fact that Abyssal Fleet ships use the English system of measurement for their gun calibers (5 inch, 8 inch, etc.), in contrast to the metric system used by the kanmusus (12.7 cm, 41 cm, etc.). If these were kanmusus sunk by careless admirals, as the fandom thinks, then wouldn't they be using the same measuring system? The thing is (and this was after watching documentaries like Discovery channel's Battlefield series), only the calibers of US ships are ever described in this way.

Add to the fact that the Fall 2013 seems to be a direct reference to the aforementioned Naval Battles of Guadalcanal (down to starting with night battles, something the IJN mainly did during that campaign), it's easy to put the two together...

Which makes one wonder if the monstrous forms of the Abyssal Fleet are simply a type of glamour to make it easier to fight the Abyssal Fleet... instead of fellow shipgirls of the US navy just doing their own missions (like, oh, defending vital airfields), while the US-aligned shipgirls think they're fending off the monstrous forces of the Abyssal Fleet...
  • Doesn't really work; the Abyssals have aviation battleships and tons of torpedo cruisers, neither of which were used by the Allies. They never even designed torpedo cruisers.

Fusing the two above theories; every ship has two parts of their soul. The grudge that they had when they sank, and their original umblemished soul. The grudge will seek out the soul of their original enemy.
Somewhere; separated by either oceans or dimensions, there is a fleet made of the souls of cute Allied Ships fighting the grudges of sunken Japanese ships.
  • The separation occurred when aliens tried to animate fallen ships. The grudges were too strong.
    • Nirvana can only occur for the ships if they seek out and find their counterpart grudge; and defeat/fuse with their Enemy Without.
  • The Summer 2014/Aleutians-Midway Campaign Event just adds more fuel to this theory, as the lines of Midway Princess and the Carrier Princess imply that they're the nightmares or dark sides of Akagi and Kaga brought to life, respectively.
    • This changes the WMG a bit. The Abyssal Fleet aren't Allied Grudges, they're the Ship Girls Grudges. Using Allied measurements for weaponry then is part of the Ironic Hell.
    • Those two specifically may even represent Akagi and Kaga's sister ships Amagi and Tosa, who were never completed due to certain circumstances. Amagi was undergoing conversion to an aircraft carrier like Akagi when she was severely damaged in the Great Kanto Earthquake of 1923 and consequently scrapped. Kaga was tapped as the replacement while Tosa was never completed as part of the Washington Naval Treaty of 1922. She was used as a testbed for armor layouts and later scuttled. Think about it. Akagi and Kaga's stillborn sisters have come back from the dead eager to slaughter the shipgirls who got a chance at life.
    • Adding credence to this theory is the Light Cruiser Dwelling Demon boss in the Winter 2015 event (Operation Hailstone). She looks like a fusion of light cruisers Naka and Agano, both of whom were sunk during Operation Hailstone.

The setting is the future of Strike Witches.
The Neuroi were Time Travellers as well as space. When they descended on Earth, they stopped World War II from happening. Oh, many of the events were the same because of their engagement; but it was the Neuroi who took the dark karma. Hitler's Time Travel Exemption Act, Godwin's Law of Time Travel. Averted. A planet united, humanity turned to White Magic and the purity of the souls of their children, and destroyed the Neuroi; and drove the shattered remnants of their flying ships into the sea.
  • The Neuroi didn't understand what they had tapped into. Time Travel is not "clean." The erased timeline always reminds reality that it had existed, and it was in the dark places at the bottom of the ocean that the Neuroi embraced dark magic; the opposite of the Strike Witches; and took it to the next level. Where the Strike Witches donned aircraft and augmented it it with magic; (and unknowingly the persona of men fighting The War to End All Wars), the Neuroi reached into the other world and took their fallen ocean ships; and amplying their grudge, became them. And the Abyssal Fleet was born; alien science, dark magic, and the grudge of the fallen all fused.
    • The magic of the Strike Witches was not enough against this. It wasn't a question of air superiority; or tactics. By definition; the Abyssal Fleet made their existence into a No Sell magically. But once again; the aliens taking the darkness of humanity from the other world and embodying it left a weakness. If a Strike Witch was willing; she could give up her humanity (but not her shining soul) and become the embodiment of a Ship Girl; the ideal of a weapon in a war that was never fought in this timeline. The Neuroi became the grudge, the Ship Girls became the personification of courage, of duty, of sacrifice.

Kantai Collection and Arpeggio of Blue Steel takes place in the same universe.

More accurately, Kantai Collection takes place after Arpeggio of Blue Steel. Sometimes during or after the war against the Fleet of Fog, humanity managed to either capture or re-create them, putting them under the control of humanity like Iona, as well as developing the technology far enough so that even destroyers and minor ships get to have their own Mental Models as well. Sometimes further after, the new Fleet of Fog, now called kanmusu or shipgirls to avoid the negative connotation of the old name (or the technology has advanced to the point they are different things), gained relatively equal rights with humanity and they are eventually assimilated into human society thanks to their new appearance. It is quite likely that somewhere in between, the nanomaterial used to build the "dummies" of the Fleet of Fog also advanced to the point it can be used to mimic human body perfectly while retaining the same destructive power.

The Crossover event in Christmas 2013 suggests that the weapons of ship girls cannot penetrate the Klein Field of the enemy. However, it takes but modifications (presumably to be rather simple, seeing that the entire arsenal can be modified without consuming many resources, and the Abyssal Fleet also get to have them) to make them only slightly less powerful than the Fleet of Fog. Combined with the fact that ship girls are stated by canon to be somewhat human in size, that means that ship girl technology is actually more advanced than the Fleet of Fog despite the objective differences in firepower. This technology is also seen in the form of repair buckets, which can instantly repair Fog ships, as well as Damage Control equipment, which can prevent them from sinking; the Repair Goddess in particular can even restore a Fog ship to full health, fuel and ammo instantaneously, at no additional cost (read: without even using nanomaterials). The Fleet of Fog has no known equipment with anywhere close to similar functions.

As for the Abyssal Fleet, they might be The Remnant of the original Fleet of Fog, or other projects that might or might not be the precursors or ship girls.
  • Crossover events aren't proof for or against this theory. Take the short story crossover fight between Kirito and Silver Crow. It's implied in-series that the Accel World setting is the future of the Sword Art Online setting. The fight, however, was straight-up portal crossover, and wasn't canon in any way that would change either setting. No Time Travel worries.
  • Alternatively, both Kancolle and Arpeggio happened on the same timeline but branched out differently; two different branches from the same trunk, so to speak.

Kantai Collection and Axis Powers Hetalia takes place in the same universe.

Aside from the fact that close to everyone is a personification of some sort (whether ships or countries), it wouldn't be surprising if the Admiral the IJN ship girls answer to is Japan himself.

Kantai Collection takes place in an Alternate History where the "Good Guys" and "Bad Guys" of WW2 have been flipped.

In this world, the OTL Axis powers-or at least Japan-are free and democratic and defending themselves against a fascist US and her allies. The Abyssal fleet is the results of American attempts to make their own kanmusu.

In the Kantai Collection world there is an enormous Eldritch Abomination under the sea.

And it doesn't care what is going on at the surface, it doesn't even bother. Everything that happens is just a side effect of ships having come to close to it when they sank. That would practically turn the whole setting into one huge Cosmic Horror Story.

The Winter 2014 Event will be another crossover with Arpeggio.

This winter season, Kancolle will get its anime while Arpeggio will be getting The Movie. Is there a better occasion to host another crossover event? Plus, Arpeggio's story has advanced to include and/or shed more light to a few more interesting new ships (Vampire, Yukikaze, Repulse, etc).

Naka in the anime will be relieved from duty (i.e. 2-4-11'd).

This will probably be revealed at the start of her focus episode which shows what happens when a girl leaves the Fleet. It'll be a heartwarming and heartbreaking story where we get to see just why she's so set on becoming an idol. We'll see the other girls' reactions as she's packing, and maybe a bit about what she does after she leaves the base. Everybody will be lonely without her. And then, for whatever reason, she'll be re-commissioned right at the end of the episode, because Naka-chan drop rate.

Gen Urobuchi has something to do with the anime.

Given how Kisaragi sank in ep. 3, which became a huge whiplash in tone up to that point, the Urobutcher has gotta be involved in the anime somehow.
  • Funnily enough, the day the episode aired, Gen officially followed the Kancolle Twitter account.

At some point, if any episode featured both Hiei and Yukikaze in the same mission, it won't have a happy ending for both ships.

Mutsuki's yet to move on
If Kisaragi's death is meant to mirror a certain show, then chances are (provided they don't sweep it under the rug) they'll also take another infamous twist, especially since it mirrors the theory of Abyssal Fleets being corrupted Fleet Girls.

Case in point: Mutsuki's unrequited feelings of love for Kisaragi is going to eat away at her until she becomes an Abyss Fleet Girl.

Re-class represents the USS Samuel B. Roberts
Re-class is noticeably younger looking compared to the other battleships-her physical age appears closer to a smaller ship like a destroyer or destroyer escort.

In real life, The Samuel B. Roberts was known as "the destroyer escort that fought like a battleship" due to her performance in the Battle off Samar. Therefore, when she shows up as an Abyssal ship, she has the fighting capabilities of a battleship, while retaining the small body of a destroyer escort.

Operation FS in the anime might not happen.
The actual Operation was a plan to cut off Allied supply lines from Fiji and Samoa that was first postponed after the Battle of the Coral Sea, then later cancelled due to massive losses in the Battle of Midway. If Kan Colle follows the history leading up to the Operation then things might be going down hill pretty quickly.