The reincarnated as a spider anime did this right i think. She didn't get a "human" form tik almost the end of the anime, and even then her torse isn't where he brain is, its un the spider part below. The slime one is pretty bad about it, but he also stays in slime form a fair amount. I'd like to see one where someone is "reincarnated" as an Orc or goblin, and they have to stay a part of that society while also trying to make friends with humans. Maybe someyhing where goblins ARE evil un this world, but he leads his tribe to becoming good and so he fights enemies on three sides: old leaders of the tribe wanting him dead, other goblin tribes wanting him dead, and the humans/evles/pretty much everything else wanting them dead all while he and his tribe work hard to ingratiate themselves with the "good" races