Since I'll never have the time to write it as a story, I'll put the general outline of a crazy fusion of Gurren Lagann, Undertale, and Evangelion that I thought up here. Needless to say, many spoilers about all the series in here.
You know how spiral power is often linked to/caused by a double-helix? Rather than just exterminating their rivals for the sake of apocalypse prevention, this Fusion's history begins with the Anti-Spirals also splitting the races into two different "strands" of spiral power. One resulting race would carry the strand of knowledge, the other the strand of life. Even if one race awoke their variety of spiral power, they cannot achieve the full potential of real spiral power and thus become a threat to the Anti-Spirals. In the case of Spiral-humanity, we get split into the normal Humans (knowledge) and Monsters (life). The beastmen were made in the image of Monsters so that even if both groups managed to get to the surface, the humans would attack them in ignorance, while in the case of the Monsters, the Barrier was created and their history erased/edited to blame the Humans, again an engineered effort on the part of Lordgenome/Anti-spirals to create conflict between the two and prevent them from collaborating to reclaim "true" spiral power, and drawing the attention of the Anti-Spirals.
Things start out with Simon falling into the Undergound when the Lagann goes nuts from Simon's insane piloting after Kamina kicks the bucket. (instead of plunging into the ravine where Nia is found) There he gets some TLC from Toriel before being persuaded to proceed pacificst-style as a means of finding himself again. As for the rest of the crew, someone else finds Nia (handwaving the core-drill key action by having the box damaged in the fall), and they all go on to deal with the trap-village as normal, minus Simon.
Meanwhile, because this is a fanfic and not a game, Simon is able to return to the True Lab and fulfill the requirements for the Pacifist end, with Lagann coming to help at the end against Asriel. More handwavey inspring bullshit about Spiral Power, and Simon uses the core drill to restore Asriel with a "let me see you grit your teeth" punch.
On the surface, Lagann picks up some distress signal from the rest of the team as they get captured, and Simon and Asriel sneak aboard the Dai-Gurren, take back the Gurren, and break everyone out. Asriel co-pilots the Gurren when they combine to beat on Guame, etc. Pretty much proceeds according to TTGL canon until the anti-spiral arc. Here is where it gets sketchy: Chara is actually an agent of the anti-spiral similar to Nia who went active too early, and didn't actually die after his soul-stealing stunt with Asriel. Chara turns public opinion against Simon by being a lookalike and doing terrible things, while at the same time Nia goes active as planned due to population. Thanks to the Monsters and their Magic, earth's denizens are able to no-sell the anti-spiral mecha she brings, but that just makes the real anti-spirals panic. They didn't spare themselves from the splitting either, and since the countermeasures made by thier "strand of knowledge" component didn't work, they send their counterparts, taking the form of the Evangelion Angels, with all the mind-screw and existential horror that implies. I didn't plan it beyond that, sadly.
And if all this doesn't make sense at all, no problem. It's just the ravings of a derpy fox all thought up shortly after beating Undertale and listening to the TTGL soundtrack.
