Doctor Strange screenwriter C. Robert Cargill has officially signed on to write the sequel. I think we all knew this was going to happen since there was news of him writing the sequel back in 2018.
But confirmations are nice since movie productions can change as often as the wind blows.
The sequel is claiming to be scheduled for a May 7th, 2021 release date and he will be co-writing with the slated director Scott Derrickson. Filming is scheduled to begin in spring of 2020.
So that likely answers the question from the Infinity War ending, Doctor Strange more than likely comes back at the end of End Game which I’m pretty sure we all assumed would be the case.
Nightmare will be the main villain in the sequel according to most news plots that have already leaked such info. His first appearance was way back in Strange Tales #110. But this book is out of reach for most collectors since it’s also the first appearance of Doctor Strange himself.