The crew of the Enterprise-D as of TNG's final season, is replacing the Atlantis expedition. More specifically, named crew, and an equivalent number of extras are going. They will have an equal incidence of the ATA gene among them to the Atlantis expedition. They have all the information available to them that the Atlantis expedition had before they went(including like, what Stargates are), and three days to prepare equipment and supplies from their own setting before they go. They are otherwise limited to what they could carry through a Stargate. They don't get the benefit of eventual contact with Stargate Earth, but we'll say they can have the Enterprise in place of the Daedalus when it would have shown up, but it's still stuck with warp engines instead of a hyperdrive (unless we think they can change that in-scenario). The couple big time travel problems will be presumed to magically resolve themselves.
No contact with Earth leaves them with some shortcomings. Can they overcome those? How will they interact with the Wraith? The other Pegasus societies? Since the Atlantis Expedition created a lot of their own problems, lets give two possible scenarios:
1. Anything is up for change. If they solve the entire series in the first episode, that's good work on them.
2. The same basic problems will at least *start* even if it doesn't make sense.