Maybe I’m a bit biased because of nostalgia, but the old engender/ageofminecraft mod was really fun and it sucks that it kind of just got swept under the rug by history. Essentially, if you crafted a mob’s fusion part and imbued it with a ton of mana and other things, you could craft a fusion, which lets you spawn an engendered mob. Engendered mobs can level up and gain special abilities as they kill other mobs, however if they die you lose them permanently and have to craft a new one. You can craft them for all mobs and bosses from vanilla, abyssalcraft, and draconic evolution. You can also give stars of valor to your mobs to make them super op.
I guess this exact design probably wouldn’t fit terramity very well, particularly the fact that the og mod has no summon limit, but I think aurukel could really cook with this. Modern Minecraft really lacks any sort of summoner build/archetype.
Actual mod overview since I suck at explaining: https://youtu.be/cgC55Id1EOI?si=G25l4_ExgBINzBdt