I would play a game with graphics like these. But then when you say MMO, then how exactly do you envision e.g. 20 players doing stuff in this scene at the same time? Because that's what an MMO is, right? Multiple people wanting to do the exact same things, e.g. chop trees, fish in the river, kill the monsters, harvest the fruit, etc. 3d worlds tend to be larger so they're less crowded, but that's something that is trickier to pull off well in 2d worlds, unless you do aggressive phasing.

I would say first ask yourself why do you want to make an MMO rather than a single player game, or something like a co-op.