Most drastic reimagining I can think of? Easy- This:
https://www.fanfiction.net/s/1079568...me-Other-World
Ever wonder what would happen if Splinter hadn't raised the boys as noble warriors? What if Shredder wasn't the evil villain we've grown to know, but a lone crusader against his cruel and vicious former mentor? The man once named Hamato Yoshi has raised four mutants as his own assassins, and only one man can defend the city against them- Oroku Saki.
I've twisted the TMNT lore around, into a bit of a "reverse world" kind of universe where the good guys aren't who one would expect. The boys in THIS world are not very nice. I'm enjoying working with a much darker version of them, especially Leo and Mikey- two completely opposite personalities, but both very damaged. Splinter's one very BAD father in this one..... It's a bit different for me, doing something so dark and yes, tragic in a twisted sort of way.
To be fair, April's acceptance isn't entirely her own idea, and I will be getting into some of the reasons behind it in more depth soon. The boys aren't the only ones in her life who are bad people, either. For anyone familiar with the original /mirage comics, Casey wasn't especially nice or a good influence all the time (he was something of a drunk womanizing bum) so it isn't much of a stretch to make him even worse.
I really took a look at all the characters and tried to figure out ways to switch their roles without changing their personalities TOO much- like for instance, Mikey is still "playful", but his idea of fun is rather twisted. Donnie is still the "geeky one", but in a very creepy stalker-ish way. Krang will still be his usual caustic self, and Baxter will still be the cowardly sycophant to Shredder, and still a bit mentally unbalanced as well- he'll just be working for a good cause this time around. As for Splinter, I have to say he's turning into a real monster in this, which is probably very counter to what most people who are familiar with him will remember. That was sort of the point, though, since this IS an "evil" version of them all.