One way SE can make ffxiv f2p is to limit those players to max lvl 20 and if a class is over 20 (from a sub ending and not renew), to have it lvl sync to 20. Doing so limits these players to try out the game to see if they want to continue.

Some other limits could be: can not join a FC, LS, speak in zone chat, no use of a retainer, no use of teleport. These are just some ideas, a lot of the P2P games have a similar F2P system in place.