Honestly, the last animated series of TMNT was the best. It took stuff from the comics and previous shows and combined it into something that worked really well. Frankly, the 'ooze' itself is alien in origin and the original comics jumped at sci-fi stuff the first chance they got. Shredder being an alien made perfect sense in the new series and made for a neat twist when he actually got beheaded like in the comics.
I'm not too bothered if they're not teenagers, mutants or technically even turtles. It's always about the characters and the dynamic between them. The last film captured this well. It's just unfortunate its main plot was a bit weak.
Not exactly the best sort of comparison, but it doesn't matter what ethnicity Nick Fury is; he kicks ass. And I'm not particularly bothered if Banshee isn't Irish; it doesn't make a difference beyond a 'token' role. Characters should be more than just basic descriptors.
That sounded way too serious for this topic.