I would add Monstrosity.

After a quest involving PoTD and pomanders, you get a Beast Pomander that allows you to turn into a monster in the overworld. At first you got three monster families (beast-, wave-, vilekin) and a base form for each (opo-opo, pugil, bee), each with their own abilities and strengths.
From here you can fight other overworld monsters to get exp for the monster family along with currency. Getting a certain level in a monster family lets you unlock new forms through quests, and you unlock new monster families. Currency lets you buy certain unique traits, abilities or weapon skills.

You build your monster using a point system, letting you remove or add traits, abilities and weapon skills. Some forms have inherent traits, like flans having higher magic power, hydra having cost reduction for each subsequent breath attack, or chimera boosting breath attacks, but cost more points.

Endgame would be Halatali. You can fight other players in the arena, or npcs. Victories give new currency that can unlock very special powers and other rewards like glamour.

And this is something that's possible to make, even with the systems we have in FFXIV right now.