Most likely XIV will be developed for PS5

Again, PS4 is likely to be dropped some time after, once it becomes too limiting for design.

XVI won't be an MMO, as it has been stated that there will not be another Final Fantasy MMO while XIV is still making money. Also, keep in mind that due to being a reaction to XIV no longer making money, it will take time after this occurrence before a new MMO has been made due to how long it takes to actually make one, so you'd be looking at quite possibly at least 3-4 FF titles after XIV dies.

As far as the main series goes, I really could see them mostly consisting of single player experiences with exceptions like XI and XIV. While the spin off titles would include the co-op adventures. Given this is kind of their formula and it has worked ever since the first Final Fantasy (Which was named such as they were going bankrupt and thought it would be their final game, but it ended up being so popular that it renewed the company due to how profitable it was)