Okay, this one won't be a real "fan fiction", I just tried to imagine how would I establish a new SG based series in the Milky Way without a reboot.
Stargate : Academy
The defense of SGC has fallen. An unknown attacker has sent a bomb what caused a terrible explosion. The whole base is being rebuilt (new studio) or the gate is moved to a new base (they were planning a Moon base anyway and they could travel with the rings and ships between Earth and the Moon). Homeworld Security will begin to construct their own jumper-type ships in Area 51 what should be able to fly through the stargates as discovery or tactical units. I imagined like a spherical unit from Jurassic World. They could have a new hangar in an upper level at the new SGC and they would use the rocket silo to fly into the stargate which can now be tilted into horizontal position too (like the Ashen gate). This new type of discovery units would be needed because the galaxy's peace after the defeat ofthe Goa'uld and the Ori crusade is gradually abolished as new power clusters and interests take control.
1. One group would begin to remove and install the stargates into underwater worlds so they can expel unkown visitors and keep these worlds just for themselves. This group could be formed by Omoroca's race (from Fire and Water), who would have allied themselves with Watergate's aliens.
2. The second group is also trying to make it impossible for others to use their own stargates, so they are moving them to space. They would be the Serrakin race and some other smaller space travelling races. Unfortunately they meet and start to trade with the Aschen, who are quite hostile towards Earth. SGC most convince the Serrakins to avoid any further contact with the Ashens and be prepared they could be invaded by other hostile races any time.
3. The third group could be the Lucian Alliance. I was never a big fan of this group in SG-1 season 9-10, but we could hear more about the foundations and what have they lost after they tried to occupy Destiny. Maybe they could make an alliance with the rouge NID guy or the main Op-Core storyline of the Stargate : Worlds could start here as well.
4. The fourth and most important is the Free Jaffa Nation, which has recently been split into several alliances of planets. While we might believe that the hate against Goa'uld and common past would bring them together, they still carry many emotional injuries from the past. Most importantly, some Jaffa Guards are NOT willing to take tretonin, because they want to maintain the larva Goa'ulds which gave them a longer life. Some of them would still hate the Tok'ra too.
5. Some smaller Goa'uld could also return who are using a new host race which are immune to symbionte poisons. They try to blackmail and weaken the Jaffa Nation with political attacks.
6. The next force could be like an invisible battlefield. It turns out that the Rhee'tou race has began to attack the Jaffa Nation, so the Sodan Jaffas started to hunt them down with their transphase technology. Maybe some Goa'uld Ashrak could provocate this conflict. Maybe the ninth chevron of the stargate could be suitable for stargates to other phase / dimensions. It seems to be a waste of time and energy that every gate has got it while they can dial only to Destiny what is not even in our galaxy anymore. Maybe some gates could be hided in other phases, so that is the reason why nobody could dial those planets.
7. The evil Asgard would fly back from Pegasus galaxy and they try to take over the Protected Planets. Nobody knows what they really want (to find their genetic ancestor or some important part of our Asgard database).
But there are so many creative way to introduce new hostile and friendly races (like the replicator destroying beam from Dakara or the Avengers virus could make a few angry and aware of the presence of other lifeforms). Maybe everyone could fight for a stargate planting ship what automatically flew back from an other galaxy to a long abadonned Ancient planet.
And who knows? Maybe there would be a new stargate from a previously unknown stargate network with a message, so the new heroes would be invited to help them or to walk into a deadly trap in a galaxy far away...
But anyway back the main characters. The main base could be like a diplomatic school type scenario, where our old heroes educate the younger generations who would try to establish some peace treaties within the galaxy. Our old heroes would return only occasionally. Jack and Sam are married with two children in Washington. Carter returns to active duty only after the fall of the SGC. Teal'c became a leader and a grandfather now. Daniel and Vala could have a 8-10 years old child too.
So I imagined this new and improved Milky Way like a new space Game of Thrones / Harry Potter / Babylon 5 combination, where we would follow a young team who would try to establish a peace treaty in this chaotic situation. New and old races, alliances, enemies, lots of gate related explorations etc.