A part of me wouldn't be surprised if they do create some new form for Mewtwo. It would be a quick way to promote the upcoming new games and the Year of Mewtwo is pretty significant too. I don't think that they'll announce Let's Go Johto games that soon at least. It would be a nice way to celebrate G/S's 20th anniversary, but it would probably be too early for them to have another pair of games ready, especially when the new generation games will overshadow them. I'm still not sure if they would make more Let's Go games. I'd be happy if they did, especially since seeing Johto with updated graphics would be amazing for me, but it just seems too early for that to happen. Not to mention I'm not sure if any other Let's Go games set in other regions would attract as much attention compared to a Yellow remake did. Plus, I imagine that they'd want to do a traditional D/P remake first.
As for Team Rocket, I'm still pretty sure that they'll stick around. Despite their more minimal appearances, they're still pretty popular characters and I can't imagine how they'd be written out after being in the cast for so long. I suppose that it isn't impossible, but it just seems unlikely to me.