Something I've seen few people talk about when it comes to current summoner discourse is how little carbuncle currently has to do during fights, which I think is a shame. Since it doesn't even cast searing light anymore, it's essentially only there to shield us occasionnaly and be a pain to summon after dying.
So my idea to remedy that and maybe bring some nuance to the job without needing to completely rework everything or even create new assets (more of patch change than an expansion change) would be to take yet another page from FFIX's book and allowing it to have different abilities based on its different gemstones, that would change alongside your own elemental phase
Currently, Carbuncle is always there in between summons, when we're using the elemental gems to cast spells of a specific element. And its gemstone / color variants only exist as a visual change. With all these gems it'd make sense to reconnect these elements.
So, I imagine that it could be elementally infused by whichever summon you last called, and change gemstone and colour accordingly, and get more of a purpose again. In addition to Radiant Shield which I think should stay, it could get a new action for each gem, Ruby Light, Emerald Light or Topaz Light , which would give the player a buff that caries out to the next elemental summon phase, creating synergy between them. It would certainly make the little guy feel more useful as well as giving players even more of an incentive to choose the order they use summons wisely.
Heck maybe the order could even then influence wether or not we get bahamut or solar bahamut as a summon at the end of the phase, if carbuncle's aether was more astral or umbral due to our actions or something.
I'm really not sure how balanced it would be, I'm no expert when it comes to that, but it could give Summoners new stuff to track and take advantage of existing assets.