Easy. Following a bit of the theme that I'm seeing in this thread, I'd take any Zelda game (2D or 3D, doesn't matter, both would be incredible options) and combine it with an MMO. Ragnarok Online, Perfect World, or Black Desert would be amazing choices.
Are you kidding me? I'm impressed Nintendo hasn't taken the plunge yet. Perhaps given their troubling history with operating a proper netcode? No idea.
Hell, even thinking about playing something like this on a handheld like the Switch makes me feel frustrated, given the ABSURD potential here.