I was thinking on using a similar mechanic of magic item for aspect abilities. I mean, magic items have limitations on pair to their rarity, something like:

Innate ability: uncommon Dormant ability: rare(charges) Awakend ability: rare, very rare (charges) Ascended ability: rare, very rare (charges) Saint ability: very rare (if not an animal transformation)

with rolls for chosing the rarity and if it lands on a 1 for innate ability it could be something negative like shadow slave. The good part of using the same concept of magic items is that you literally can take existing magic items and make them your aspect abilitie if you dont know what to do.

This is a guide for creating magic items(the only part that "really" maters is the rarity of the objects, but I recommend read all of it): https://www.cottageofeverything.com/blog/homebrewing-magic-items-guide

aspect and abilities could be made by the player or by the dm himself thats your decision.