Carbuncle could do 50 potency attacks, like the Baha/Phoenix, whenever the summoner attacks.

If Carbuncle was decoupled from the other summons it could allow for stuff like being able to trade positions with the carbuncle. Like on a 20s cooldown. Which could be useful positioning.

Replacing Ifrit, Titan, and Garuda with Leviathan, Ramuh, and Shiva after a Phoenix. So I/T/G are umbral and L/R/S are astral. The summons could keep their initial damage potencies just like how phoenix still does 1,300 potency attack like bahamut's. And it wouldn't just be a cosmetic change, as the new summons would have to have their own boons they grant which can change the playstyle.

Could have during the 'astral gems' a trait that changes resurrection and physic. Like resurrection becomes Mog Create. Costs like 4000 MP, 60 second flat internal cooldown, instant cast resurrection. As that would alleviate a tiny bit of summoners burden. Physic could be Pom Cure and actually uses INT to heal with, could work off a charge system, like having 2 or 3 of these Pom Cures. Both Mog Create and Pom Cure showing up near your buffs section with a timer that starts at 60s.

Energy Drain, Fester, Painflare, and Energy Siphon are harder to pin down, but since their nature is to weave them them in with the rest of your kit. Perhaps they can bolster other aspects when weaved in.