Right now custom mobs for ctms are in a nice spot, but in the end all bypedal mobs that can be customized properly with heads and armor are all undead. This was a problem even in the past but now more than ever with undeads not being able to swim (except the drowned) the solution i'd like to propose is to add a new mob for mapmakers only.
The mob would use a player modle and would be able to render armor + have different skins that you could add to it using the same method avaible for custom heads, it would also be able to both use normal weapons, bows and tridents, and for the sounds the normal player ones (Customizable sounds with Nbt would be nice but both work!) Size would also be nice to be able to be modified but that could be hard to implement so we can do without it.
I feel like something like this would change mapmaking alot and for the better, the methods we have right now for both normal mobs and custom npc feel really hackish and this would make maps alot more clean without needing resource packs or anything similar, while also improving alot PvE maps like Ctms or any kind of adventure style, making them feel more interesting and less relying on the use of Smite weapons.