Well.... it's sort of hard since this is an MMO and all my favorite games of all time are Single Player (for the most part).
Like, I love Zelda but I really don't think it would translate that well into an MMO (at least one where you are the hero). Maybe something could be thought up.

Or like SMT. There IS an SMT MMO, actually, and it was very good but not popular in the west so of course the western version was cancelled. :'(

Hmm what else? Pokemon? Yes Pokemon MMO please. But that is, and would be, a completely different MMO subgenre.
Guilty Gear? It's a 2D fighting game, it wouldn't translate well to MMO.

So I guess my answer is "IDK" because genres are so different.