Hard to really call Pokemon Go an "MMO" it's a lite version for sure but I agree that a fully fledged Pokemon MMO is an untapped market.

The older Pokemon games had some great mechanics where at least duo'ing could easily be a thing for PvE content and obviously the whole PvP aspect would simply be what we have today (except wandering the world).

I think what? TemTem is the only real one in this space? I haven't played it so I don't know how good it is but perhaps Nintendo might one day partner-up to offer a spin-off if their engine is decent enough.