This is exactly where I think the current idea of "incomplete" SMN lies, is that the rotation after Demi-Phoenix is exactly the same as Demi-Bahamut. the sole difference being that instead of intense damage from Bahamut, you just do a lot of regen in between semi-high numbers with phoenix. everything in-between is Ifrit, Titan, and Garuda spam. hell, Leviathan, Ramuh, and Shiva could probably fit as support AoE summons in comparison to Ifrit, Titan, and Garuda being DPS AoE. Leviathan's Astral flow places a debuff on enemies, every time you attack with Leviathan Rite/Catastrophe it makes an explosion that damages and gives minor healing. Ramuh's could be a DoT/Damage Down debuff that increases in potency depending on how many Rites/Catastrophes you use beforehand. Shiva's could be a shield that explodes after it's broken or on command. and once again, why stop there? continue on with Trances and summons for Heavensward and Stormblood, maybe they can be specific single-target variants, one set for outright damage and one for more support. upgrade the ARR primals to their Eden variants with potency increases for no DPS loss later down the line. add in extra gauges or mechanics to do off-hand summons for powerful attacks or supports with summons like Odin, Knights of the Round, Moggle Mog, hell maybe even Eureka too, after using your Demi-summons in specific orders. maybe have carbuncle work on the old Demi mechanic with attacking between pet commands and being eaten for summons, could possibly move Ruin IV procs to that if there's enough time that it's on the field between summons. Aetherflow stacks coming from your summoning instead of Energy Drain/Siphon so there's more Fester and Painflare weaving. maybe call upon the Eldest of Elder Primals (you know the ones) as a reward for a specific weaving of Demi-summons and keeping Astral or Umbral aether flowing without opposition.
there's so much to be built off of new SMN IMO, and the template for it is easily already there just in the main story alone. add in all the side story primals and the like and you got more than enough to work with for at least 4 expansions of steady additions and improvements, if not more.