While it’s not the answer people usually like to hear, the chances of an official Pokemon MMO coming out in the near future are pretty low. Ultra Sun and Moon sold roughly 1.2 million copies over its first weekend in Japan alone, so it’s not likely they’d want to change a formula that continues to work so well for them.
As for USUM updates, I won’t say it’s impossible, but I wouldn’t expect to see any. With Game Freak switching their focus (no pun intended) to the Switch, any updates/DLC for USUM would certainly be a surprise.
Oh, and since this about the games rather than the franchise as a whole, I’m going to move this over to our Pokemon Gaming Central board for you.