Why not those two? They are kinda a staple of undead Creatures. And you did not mention Phantoms, the Wither or the Ender Dragon, so are those fair game then. XD
Mobs will be very RNG, so maybe the one in a billion chance happens, a plant based mob that looks like a dick and explodes when it comes close happens to live on an Island closeby. XD
Yeah moving Block Grids are planned in some way, but they are a “to do later” thing.