The Vigil Codex isn't just made up of incoherent roaring
The description of Sven's Warcry shows that he calls lines from the Vigil Codex to grant him and his allies morale. But in-game, all Sven does is let out an intimidating roar. But the Vigil Codex belongs to the caste of knights that Sven should hate, according to his lore. So what's really happening is, he reads the lines of the Vigil Codex to himself, and much like what happens when Bill O'Riely hears someone say "The Theory of Evolution is a plausible theory", he gets pissed off by what they say. This fuels his rage, and like with his ultimate, this is channeled into energy to kill his enemies with.
Arc Warden's lore will somehow be related to Visage and Razor.
Visage and Razor are both guardians of a maze in the afterlife, with Razor policing it, so it stands to reason he'll be a warden of some sort. In addition, he also attacks with lightning like Razor.
The source of Anti Mage's abilities really aren't using the magic he wants to eliminate
- One of Anti Mage's gaping holes in his character is his presumed hypocritical use of magic in his abilities (his blink and mana void) despite having an agenda against magic. However, there are possible explanations that his abilities, however magic they may seem, are not explicably 'magic'.
- His status as a monk possibly suggests that his powers draw from Life Energy, or something equivalent, and that abilities using them are more or less Ki Attacks. This would imply that his power stems from physical mastery, and not magic.
- Anti Mage's hatred for magic refers to a very particular and difficult to interpret class of magic. As in the sort most mages use. It's not certain what could possibly be the boundaries for this class of magic, but Anti Mage's abilities fall outside of them, one way or another. This might also apply to the store's enchanted items, that may also not be strictly 'magic'.
There isn't one Final Battle - all the battles played in real life actually do happen in-universe
- From the developer forums, the Ancients bestow "kinetic energy, mana, protection, even resurrection" in addition to influencing the surrounding landscapes and civilizations. It's not unreasonable to assume they can warp reality in other ways - like, say, creating illusory copies of themselves and their towers. Each side's forces perceive that they are destroying towers and bases, but the actual Ancients go untouched. This reality-warping can also explain how nominally Radiant or Dire heroes fight for the opposite side (that they sometimes do is acknowledged in-game in the item description for Morphling's Radiant Crystal Bracer cosmetic). Since souls exist in the DOTA universe, it can be assumed that if the Ancients can get a soul before it goes to the Underscape, they can resurrect its owner. But if a hero dies and their soul is claimed/resurrected by the opposing Ancient, they would end up fighting for the other side. So to sum up, the Ancients are powerful enough to endlessly create perfect decoys of the battlefield, and they constantly snipe the souls of the other side's heroes. Thus, every battle fought in real life has actually taken place in-universe.
- In any case, it is stated in the lore that multiple Radiant and Dire shards fell after the moon shattered; there could, indeed, be thousands scattered across the world, and each battle in-game simply takes place at a different crash-site, with heroes wandering the world to reach them.
The Fundamentals literally represent the four forces.
Not much of a guess, given how heavily it's implied, but here are my justifications:
- Enigma: He's gravity. His ultimate is called "black hole" and his lore states that he might be one.
- Keeper of the Light: Electromagnetism. Light is electromagnetic radiation.
- Io: The strong force. Its lore describes it as holding particles together. (So does EM, but that fits Keeper of the Light to well to give it to the Wisp.)
- Chaos Knight: Has to be the weak force, which sounds a bit odd, but remember that the weak force governs some forms of radioactive decay — an inherently random process.
- More likely, Io is a generic representation of atoms (Spirits are electrons, Overload for radiation, tether for bonds, Relocate for quantum tunneling) and Chaos Knight is a representation of Entropy. It's worth noting that Fundamentals are basically stated to be embodiements of laws of nature, so they can be a little hard to pinpoint.
- On the other side, Sven has these 4 fundamental forces in a way. Storm Bolt, as the name says is EM force, Cleave makes sense if you oppose strong force, Warcry is some some sort of RADI Oactive power and God Strength, well gravity is also known as relativity. Also, Omniknight works in a similar way, but the "Religious" (His skills are named: Purificating Light, Repel, Degen Aura, Guardian Angel) sort of stuff, rather than Sven "SCIENCE!".
Troll Warlord is actually secretly a nice guy.
But of course, trolls being trolls, Ja'rakal got nothing but humiliation after humiliation from his fellow trolls for daring to be different. Ja'rakal, suffering from a severe case of insecurity and inferiority complex, he decided to merely act like a major jerkass in the belief that it would bring him some respect from his fellow trolls. It turns out that he was too unpleasant even for them, getting him kicked out. He slew them later out of either anger or thinking that he wasn't enough of a jerkass (or even both). The reason he keeps acting like a jerkass to everyone else is because he believe that's the only way to get respect.
That final battle happened long time ago.
Gameplay is those two guys from comicbook arguing with friends at the bar 'Who would win'. That explains things such as two Rapiers at once: for the sake of argument they make an assumption 'what if Drow Ranger and Windrunner fought each other? And if they both had THE Rapier?'. This also explains inconsistency of having 'good' hero on the 'evil' side. And how supposedly unbeaten heroes are beaten again and again. This also means that Sentinels won in the Final Battle.
- This also explains things such as why Gyrocopter or Viper would need boots, or all-mighty powerful fundamental gods like Enigma or Chaos Knight being beaten by a tiny midget with a sniper rifle.
The Ancients reduce heroes to a basic state before every battle.
In the lore, most of the heroes are described as being incredibly powerful – elite warriors, brilliant mages or godlike elemental beings. In-game, these descriptions are fitting for a fully farmed level 25 hero, but not for a level 1 hero, who can easily be killed by a single tower or a few regular creeps. Why are these incredible heroes so weak to start with? Because the auras of each Ancient strip away the power of opposing heroes at the start of each battle as a kind of sabotage; thus, friendly Ancients have to compensate by giving them an aura that lets them level up and magically gain gold.
Pugna was transformed into whatever he is at a very young age.
And that is why he is so short, as his body hasn't aged since then. His voice matured normally for some reason however..
- Pugna became what he is when Viper, well, Vipered him. Could have been when he was young though. But regarding what he is, I assume he isn't undead, but more like animated dying. Viper's ult (which is what was canonically used on him) deals high damage over a fairly long duration. I can imagine Pugna using nether magics such as life drain to prevent himself from totally dying. Because of this lack of flesh and poison would explain his ABYSMAL strength stats and decent movement speed in short order.
The "truth of the universe" that Leshrac discovered was that the universe is, well, a video game.
Nerf Now brought this theory up as a joke, but it's an entirely reasonable idea.
The notion of existing purely for someone's else's entertainment
has been explored in fiction before, and not everyone takes it well.
Valve has a lot of potential power to damage the Dota 2 universe, having the power to weaken even the Fundamentals (io tether nerf) or create chaos at a moment's notice (Year beast event, Wraith-Night) and fears that they'll do something even worse. This is how Outworld Devourer knows about game terminology (such as his line for killing Tinker), and the reason he alludes to knowing what's going to happen is because he's been in many a Dota match before.
Windranger needs to use wind magic to hit her targets.
Windranger is usually played more like an agility hero than an intelligence hero, so why is INT her main attribute? Because unaided, she's only a middling archer, and so she's actually helping her arrows along with minor wind magics.