It will be some time from now if it happens (4years+) but I think another expansion is possible if their subscription base holds steady. When Seekers of Adoulin was released a decent chunk of players come back to the game to try it out. Unfortunately they made very some bad decisions. Specifically, trumping the mythic/relic/empyrean weapons (which took millions of gil and months to create) with weapons that rained from the sky.

They have recently started introducing loads of QoL improvements that the players have been requesting for years. The Jobs classes are also getting some very considerate balancing. The classes have been unbalanced since the game's servers went up. Given these improvements it would be a shame if they did not make another expansion in an attempt to grab players back to a game that has been knocked into shape.

tl:dr: Maybe, not sure, possibly, I hope so.