The most fun I've ever had online has been exactly when you take complicated, typically singleplayer scenarios, and put them in multiplayer.
If you've ever flown IL-2 Sturmovik: 1946, did you try Lowengrin's dynamic campaign generator? It used to have moving frontlines with ground troops which could capture air fields, persistent squadron aircraft, etc, etc, and randomly generated missions. Everything you did, like bombing troops on the ground and shooting down aircraft, would have an actual impact on the war. It was great. And then, some guys made a MP server out of it. All of a sudden it mattered which side you were flying with, and you could come back the next day, see the effects of other people's sorties, and try to push back the enemy yet again. It was great fun!
Another amazing experience was flying with Ghost Skies. It's basically an online campaign with two factions. Before each mission, commanders set orders for all the task forces and armies on the ground, and they move along the predefined paths, etc. This happens a couple of times a week. And of course, everything is persistent all over again. So now there is a big, community/player controlled strategic layer on top of IL-2. Furthermore, getting shot down meant you were out of that week's mission - your scores would be reset (and the more you lived, the more score you would get) - and you would lose an airplane for your faction, which could severely impact performance in the long-run. So people really tried to stay alive!
Both of these were an absolute blast. I wish I had any friends who were into flight sims, but given that I don't, organised MP with voice comms is what I prefer.
Have you tried Flaming Cliffs 3? That seems to be an essentially MP offering, which can integrate with most DCS study sims. I wonder how that is