This is a simple mod request, and would basically just be for texturers who want to do this kind of thing. To get to the point, what this mod will do is change all of the models for mobs to be simple humanoids, just like the player's model or zombies or skeletons. They would still behave the same (slimes hopping about, spiders climbing walls etc.), and if you could, have the same sized hit box and shape (physical shape for moving about like silverfish being tiny) so they are still limited like normal and silverfish don't suffocate when they spawn. If you will make this mod, I'll get on the default skins immediately.

Side note: I would also like a mod that adds hat layers to various mobs that don't have hat layer availability on their skins (namingly creepers).