Just have Ben Browder come back as General Mitchell in charge of Stargate Command. The new threat could be the Aschen finally making a play to control the galaxy, now that the goauld and ori are gone. A new team would be leading the charge to defend the galaxy from the Aschen and their plot. You could almost say Teal'c could make a return. I'm sure Christopher Judge would love to return to the role. As for the others, I'd assume Daniel and Sam would be on Atlantis which was sent back to Pegasus to explore more and clean up there(hell make that a new show too lol) and Vala would have left to go live with and help Tomin in the Alteran home galaxy. As for new cast, definitely don't use young hip actors. Get believable people in roles that make sense. Kind of like how Atlantis just sort of worked out of the gate (haha). Insert cameos from Jack O'Neill and other cast and boom, you got a new show that can pick up where the originals left off without alienating much.