Glamours have been mentioned twice already, so instead I'll say The Companion System still has a lot of untapped potential. And especially since Yoshi is trying to make this game more and more solo friendly... maybe they could... you know... expand it~

You can spec your bird out to function as one of the 3 roles. But it'd be cool if, instead of just linearly picking up every ability, you had skill trees so your bird could specialize at healing a specific way, focus on DPSing differently, etc. That way you can build it to best compliment what class you're playing or what you wanna do with it
- AoEs for fighting groups
- single target focus for fighting Fate bosses
- a ranged spec to better avoid enemy AoEs
- etc