So, I'm not sure if this is the right place to put this, but what is the best possible thing that could happen to the main TES series due to TESO?

I'm not sure. More fans who don't know much about the lore - not a good thing.
Adding multiplayer - not a good thing.
Using the same art and design of provinces for next TES as in TESO, in the meaning not trying to make it from scratch by themselves - the worst thing.
Using feedback from TESO fans in developing 6th TES - very bad.
I can't come up with anything good. MMOs are meant to be fast-food of the game industry, with some exceptions.

Art assets for other regions could help push larger worlds? TESO is using assets from Oblivion and Skyrim after all.