Personally there are couple of things I wish they would do with summoner instead of what we have right now.
Having Enkindle work like it used to back in the freaking live letter with two stacks you have to time properly to use during Summon Bahamut/Phoenix. I mean seriously, the 1k potency was not worth it for this change.
Having the Gemstone summons be oGcds instead of Gcds, not like Ruin has a free weave window all the time. Would also open the rotation slightly more in case of downtime and give Ruin 4 a better purpose as a proper movement and weaving tool.
Giving Titans spells cast times comparable to what Ruin already has, alongside slightly weaker potencies make it the middle ground between Garudas insta cast but weaker spells and Ifrits long cast but stronger spells.
Having Summon Bahamut/Phoenix be something you build towards via aetherflow and gemstone summons. Something not to dissimilar to StB Bahamut where you had to build up Dreadwyrm aether via aetherflow spells to summon Bahamut just using aetherflow for the gemstones. Sure it would mean no Bahamut in the first minute but that would be a small punishment for some depth. It would also make Energy drain into something more then just a superfluous 60 cooldown and into an actual mechanic.
In practice it would be something like ruin -> Dreadwyrm trance -> energy drain (gives gemstones/aetherflow) -> gemstones (each gives stack of astral aether could need only two to summon Demi’s to reduce rigidness of downtime) -> Summon Bahamut -> energy drain -> gemstones -> summon Phoenix -> energy drain -> gemstones -> Summon bahamut -> repeat.
Stuff would be modified, like potencies to make sure you want to use dreadwyrm trance instead of holding it for later, perhaps swap stuff around, things like that to fit with the two minute burst windows.
Bring back the old Firebird trace mechanic of using Fountain of fire into Brand of Purgatory. This was a cool mechanic that's just plain unique honesty. At the least, they could have Fountain of Fire buff Brand of Purgatorys potency for single target situations and keep the one button Brand for Aoe.
For smaller things, the Gemstone Ruins/Outbursts should gave their elemental affinites shown off better. It doesn't need to be a lot, just a basic explosion of Wind/Earth/Fire when the spell lands, and have those spell icons change colors. Give the Carbuncle auto attacks again, I already have a cheerleader minion, I don't need it as a pet, and finally bring the Elemental Astralflows to a lower level with weaker potencies/no wind puddle/crimson strike.
If they went through with these originally, I would honestly have a lot less to be upset about because it would have at least been more then mashing 2 until it’s time for bahamut.