Some Digimon have connections with human gods/goddesses.After all, Crusadermon has an attack named after Athena, and Nefertimon has an attack named for Isis.
Kouji and Kouichi are really the same person.This is one I've seen around, where they discover they are truly halves to a whole and combine to become one. Not creepy at all....
The four lost kids would be chosen to be the wielders of the four other element spirits, if these weren't intercepted by Lucemon and his lackeys.There's no other reason on why they should be there, and why they had personalities that briefly matched the missing spirits.
The reason Kouji is noted to experience more pain during his Beast Spirit transformation as well as when de-transforming is because he is picking up on Kouichi's pain.When you think about it, digivolving in Frontier involves pretty much layering a new spirit made of data (and therefore, according to the laws of the digital world, a new physical form) over your own. This would bring you (for the moment in happens in the very least) much closer to the spirital relm than normal. Since twins are noted, in both fiction and real life, to have a sort of mental link whether they were raised together or not, it is entirely plausible that this temporary shift away from the usual physical bounds would cause Kouji to pick up on Kouichi's physical pain from his fall (and later, his mental anguish at being, well, dead and brainwashed). To further support this, if you look you'll notice that when the others are affected by their transformations it's generally in the form of being out of breath and tired. The first two times he changes back, Kouji is shown at least twice to be clutching his chest. As the first happens within roughly the first few hours of being in the digital world and the second the next day, when you allow for the usual bit of digital world-real world time dissonance this could eaisly match up with Kouichi's fall and then the defibrillators used on him and/or his death (time loop stability aside).
Snimon is Myotismon reincarnated.Just listen to his voice in the dub. Not only does he have Myotismon's voice, but even his sadistic personality.
Shamanmon actually predicted the Koichi arc.
Mercuremon's minions were fake Digimon.None of them release a Digi Egg upon being defeated.
The ancient warriors sacrificed themselves because they were going to die anyway.In V-Tamer 01 it's said Ancient Digimon's short lifespans caused them to go extinct. While this rule isn't always followed there isn't any evidence of Frontier not following it. When the ancient warriors saw the destruction new Digimon like Lucemon could bring they decided to give up their bodies and live on inspirit, to protect future generations. Human hosts were chosen because giving the spirits to other Digimon would curse them with short lifespans too. Humans are immune to the side effects because we age through cell division and think with our brains, not through overwrites.
The other evil spirits were corrupted, not just Kouchi'sThey barely look anything like the Ancient Warriors.
Zephyrmon is not a Beast Spirit... she is actually a human spirit, possibly of something such as aether the substance that filled the sky (which was different to wind in classical mythology). This explains why Zoe didn't struggle to control her the first time and why she looks so humanoid. Alternatively...
Zoe's control of Zephyrmon is thanks to CalmarimonIf I recall correctly (admittedly, it's been a while), Zoe's the only member of the group who first called upon her Beast Spirit in combat conditions. This is important, because the initial invocation involves a surge of primal emotion (fear, aggression, etc.) that usually causes the just awakened Beast Spirit to briefly go berserk. Zoe, however, was able to channel that surge into beating the crap out of Calmarimon, thus the illusion that Zoe gained easy control over Zephyrmon.
The in-training Digimon (except Tsunomon) at Togemon's school in The Odd One Out are the main Digimon from Tamers after they were sent back to the Digital World
Murukusmon is a Fallen Legendary Warrior.I base this on the fact that he is able to slide evolve between his Champion and Ultimate levels, Darcmon and Hippogriffomon respectively, like the Ten Legendary Warriors, who are the only Digimon ever shown to be able to do that.
Unified Spirit Evolution mechanicsAfter some thought, I think I've figured out exactly how this works, down to the most minute detail. These Digivolutions involve five sets of spirits. Might seem like an arbitrary number on paper, except it isn't: the reason that five are used is that it leaves room for two sets of complementary elements: Human and Beast spirits, further separated into Flame and Water, Earth and Wind, Ice and Wood Light and Darkness, and lastly, Rock and Steel. These pairings have some levels of reasoning to them: Light/Darkness, Flame/Water and Earth/Wind are obvious complements to each-other, while Thunder/Steel are merely poetic terms for modern electronics and Ice/Wood more visibly represent natural phenomena, further adding to the duality theme. Let's look at the door... I mean the Unified Spirit Evolutions performed.
Trailmons are later extinctedIf Frontier is supposed to be a prequel of Adventure, it may mean that after Frontier they became extinct which would explain why they don't appear in any other season anymore.