My idea would be this:
- At base value you summon the different coloured Carbuncles, these aspect your Ruin. No more timers, Ruby is your long cast strong damage, Emerald is your instant cast mobile and Topaz is your AoE. You can switch between these at will and they use their on summon ability as a bonus for summoning them and of course eventually you can glamour them into Egi.
- Aether flow becomes something that generates over time, energy drain becoming a means of forcing it.
- Eventually you get aether charge when you become a summoner, which brings in the normal mechanic of summoning Egi as you unlock them and getting their special spells.
- Once you get Bahamut/Phoenix, aether charge goes away as before and eventually your Egi go away to become the full sized primal Eikons - perhaps this should be a thing a bit earlier.
- Things change up when you get aether empowerment, where you can use aether flow to empower your summons so Bahamut becomes Solar Bahamut, Phoenix becomes something like Lunar Phoenix.
- Further aether empowerments could let you summon other Primals like Ramuh and Leviathan as the aether empowered versions of Ifrit, Garuda and Titan.
With these changes I wanted to make summoning as Summoner feel a bit more involved and tactical, a job that's easy to pick up but interesting to master. You can still have one Carbuncle out, empower Primals willy nilly and still do something decent, it's still the job that's not going to punish you on the level a kob like Black Mage does, yet there's room for optimisations. Which Carbuncle/Egi are you using for their unique Ruin? Which Primal are you empowering? Is it worth empowering Bahamut into Solar Bahamut for your burst or do you want to save it for another primal for an overall gain in DPS? I also wanted to try to link up the aether flow mechanic with the rest of the kit, as it always felt odd to me how disjointed the job is due to having two mechanics that run entirely separately from each other when most other jobs have all mechanics flow with each other. This makes it so you can use aether flow a bit more often and more tactically at higher levels, while also having an 'oops I screwed up and spent all my flow charges right before aether empowerment came off cooldown' button in good old energy drain.
Bonus, up to five extra Primals if you include Solar Bahamut!