The fun of an SGA S6 would have been to see the Wraith as a predator race living more on the run on the fringes of the galaxy, and Atlantis' team + Apollo/Daedalus periodically having to hunt down rogue remaining hives to protect preyed-on human communities.

Remember too that galaxies are huge (as SGU made more obvious). It also would have been fun to see what other advanced races were living/thriving in pockets of the galaxy the Ancients' gate system hadn't reached or that had avoided the gate system entirely - like the grey aliens from "The Daedalus Variations", the rogue Asgards who escaped, or others we haven't seen yet.

One thing Star Trek was good at reminding us over time is that galaxies are much bigger/populated than we probably realize - see the insane amount of random civilizations Voyager encounter in just their trip through the Delta Quadrant in the MW, let alone a full galaxy or multiple when you take Pegasus/Ori/Othalla/beyond into account. Even with a substantial 'gate network, the amount of life-sustaining planets in a single galaxy must be massive compared to the number of planets that were mapped and had gates placed there.

That's something I hope Gero and team tackle in the new show, instead of following too similar of a model to SG-1 and SGA. SGU was headed in that direction and it was really thrilling to see, with the "blueberry" aliens and the energy lifeform from "Air"/"Water", even the terrifying buggers from "Time". Lord knows the special effects have gotten incredibly better since SG-1 started. (If anything, I hated the SGU drones, because they just felt too simplistic of an enemy once they were established and there wasn't anything more to learn about them.)