Most likely I would expect Shiva/leviathan/ramuh next expansion. It's the logical evolution of having them replace ifrit/Titan/Garuda every demi summon. So your rotation would be bahamut--- ifrit/Titan/Garuda---- phoenix----shiva/ramuh/leviathan. That seems the most likely path they'll take next expac. It also nicely covers all the elements.

Overall while I do miss some of the complexity from previous SMN i can accept the simplicity of this version as a building block. My main complaints with the job in it's current iteration are demi Simmons being effected by spell speed, having them be 56 seconds instead of 60 drifts the use of aether flow thru the rotation which while not a major issue is a minor annoyance. And honestly aetherflow just feels out of place now, I wouldn't be surprised if they completely removed it next expac in favor of something else.

I would hope to see a bit more complexity added next expac to the frame that they've built this time. Also I've been saying for years that they need to give physick an upgrade to be a summon mog ability on a cooldown, some good old FFT feelings while making the skill actually somewhat useful. Making it an ogcd heal on a long cooldown would make it useable without stepping on red mages toes as the utility caster imo.