Im pretty sure they are just going to go the BLM route and divide the 6 summons between Umbral and Astral elements (3 each respectively). It would keep the same gameplay design and allow for more freedom i.e. you can choose which form to start in.

Example:

Bahamut (Umbral) - Shiva, Titan, Leviathan
Phoenix (Astral) - Ifrit, Ramuh, Garuda.

Plays like EW SMN but you can choose to go into either Bahamut or Phoenix at the start of the battle and it simply flips to the other once you do the full rotation. Then once you complete both trances you then unlock Judgment Trance which lets you summon Alexander. However I would like to see them have fun with SMN. Like a random chance to summon fat chocobo or even king mog with only visual differences but otherwise no change to potency or gameplay.