I don't see why FF17 should be the next MMO game in the FF series. Just because FF11 and FF14 has two singleplayer games between them does not mean it should be the same for FF14 and FF17
However, let's say there is going to be one. I'd imagine it different than FF11 and FF14 because that doesn't work and is a waste of resources and time and can results in FF14 1.0 failure all over again.
Maybe a mix of tab-targeting and action-combat if not pure action-combat with an actual open world ( Open world as in WoW where you can travel from a zone to another without loading screen in-between )
The game might also adopt the "Player-driven economy" concept but without being a PvP-focused game. As well as introducing new concepts for it to stand out the rest of the MMO games.
Speaking of PvP, the new game might look to where FFXIV did a poor job regarding the PvP. but would prefer that to happen without introducing open-world PvP.
That's all i can think now of a new FF MMO game. If i was not mistake, they mentioned in an interview that FF14 might receive an engine update, so i guess that means FF14 is going to stay updated for quite a long time before SE decides to make a third FF MMO