My take is they should let warlocks buy(or quest in a perfect world) some kind of soul gem that would function like a drain soul ripping out the essence of the demon you kill, which can then be infused into your demon. Maybe via end quest that takes place in a ceremony in undercity? My mind always goes there from binding my first demons but maybe with more binding runes and npcs helping with the infusion process
Imo theyd have to split up the demons into families like how hunters have for pvp reasons.
They could do some real cool thematic stuff but I’d settle for a lazy fix with some glyphs for new models.