SGA - there was a planned season 6 where Atlantis ended up going through time, then to the triangulum galaxy and more time hijinks. Atlantis Vs Atlantis and peace in the Pegasus galaxy.
You can find it on the josephmallozi blog as Stargate Atlantis virtual season 6
SGU - had so much potential and season 2 was good. It ended with the crew going into stasis to travel the gap between galaxies...
SGU would definitely be my answer. You couldn't ask for a better show to do it with either, because of the way it ended it would be easy to have had problems with stasis to explain why some of the actors didn't come back or even the aging of all the ones who do come back. So easy to write around the long time it was off air and still pick up and run with it.
Stargate's one of those fun series where even the original series is less like it needed another season, and more like they needed more of some of their existing seasons to let the stories really go where they had (mostly) wanted without the kinda rushed spots here & there.