This can take place anytime after Cordelia "ascends" in Season 3, though I'd prefer it after season 5 when she "dies." Pretty much, Cordelia gets frustrated with just watching from on high and manages to get the PTB to send her back down to help, only she ends up with the Stargate Crew (Which ever series is up to you) instead of with her friends with no memory.
Cordelia cannot remember anything when she first "descends." How she recovers her memory is up to you.
Cordelia still gets visions. Painful ones that aide in whatever the SGC is dealing with.
The PTB ...