Depends on the strength of the branding and how much of a cash cow it continues to be.
My predictions, from what I'm pretty sure of, to the most banal, on how to stretch FFRK out:
-6* equipment (the character's ultimate weapons, etc. Yeah, I think we're gonna have a bunch of Ultima Weapon [insert realm here])
-Storylines for the core games wrap up
-6* abilities become easier to farm
-Memory Crystal 3, characters that get this break the 9999 limit, up to 99999
-Side games start (FFT, FFX-2, Crisis Core, TAY, etc) and are absurdly difficult to begin with. Maybe characters like Cecil, Zack, Yuna, Lightning, etc. have unlockable costumes that change their RS to these new realms. Dr Mog says "we've finally restored the records, but although we solved half the mystery, we need to go deeper" yadda yadda yadda. But since this either applies to -some- Realm Synergies, or more likely their own (with some equipment retroactively being changed e.g. Zack's sword), it leads us to:
-Difficulty above Elite is introduced for History dungeons (paintings). Ties in to the 12 storyline bosses and Tyro's main story (Perhaps they're sequentially unlocked by the number of the 12 storyline bosses you kill. So it takes 1 year minimum to 'beat' the game if the rotation for the storyline bosses don't change). Goes up to ~190 in level. Even the first few e.g. Mako Reactor 7, Zanarkand, etc are absurdly difficult anyway till at least r3 in a few 6* abilities.
-Events system revamp, because it's become a bloated mess of more and more tacked-on tiers of difficulty plus needing to cram in way too many characters and their MCs and all that jazz into the current Challenge event format. Again, can tie into Tyro's story.
There's many ways for FFRK to stretch comfortably to the 3 year mark. I mean, the 12 bosses alone already guarantees content for the next 11 months, although how meaningful progression/difficulty for those will be remains to be seen (I don't know if having them all at the same 'tier', or making them increasingly more difficult is a better choice because both have their downsides)
edit: For reference PAD is 3 years 8 months and going strong. The mudflation slowed down but is still significant over the past few months.
edit: Or just as likely, after a certain point, you beat Tyro's story, DeNA are all "Thanks for playing!" and events just go on a rotation for perpetuity with the same gear/characters rotating in and out just for the stragglers to "beat the game", then they start fresh with another Final Fantasy mobile game, with a clean slate so they don't make the same design mistakes, and start it all over again. (I mean, if World of Final Fantasy can do it with the world of Grimoire, why not?) And have Cloud be the first character you recruit because Cloud.