Either:
  1. Level Squish and Stat Squish (similar to what WoW did)
  2. A stat squish (like from endwalker)
  3. No more levels or higher item gear (like GW2)

As we are starting to approach that upper limit for what the system can handle.

For example lets go over the job stone part of the inventory. If you factor in the crafter stones, and have unlocked each of the current jobs available, there is exactly 1 slot remaining, and we already know that the beastmaster is on the way for this expansion to occupy that slot.

Which speaking of beast master and therefore limited jobs, brings us to our next related manner: Eventually blue mage would reach level 100 if the cap is 100 in just one more expansion after this one, and however many expansions it would take for beastmaster to reach cap.

Final Fantasy 11, the predecessor mmorpg to 14, it's level cap is 99. And that evidently started to break things as FF11 for awhile was designed around level 75 cap.

Although what can be done, similar to FF11, is Master Levels. Levels where you get exemplar points and that results in stat benefits for maintaining those levels, with a cap of 50. But if you die, you lose some exemplar points and this will result in your level decreasing again. Similar in FFXIV to the gaining and losing of Eureka levels.