- Barbarians of Lemuria
- Forbidden Lands
- Hillfolk
- Mythras
- Lancer
- ICON
- Eclipse Phase 1e
- Worlds Without Number
- Shadow of the Weird Wizard
- Swords of the Serpentine
There are probably a lot more that I'm forgetting at the moment, but these have been on my mind a lot lately. Everyone's gravitated toward D&D5e for the last ~10 years, so I haven't had a lot of chances to play in or run campaigns in many other systems.
Give me something like The Matrix Online again. I want missions/quests that are unique. Only one person/team can complete them. Failing is an acceptable outcome and has ramifications for the rest of the story. You would absolutely need writers and asset artists on staff as old things content gets completed. Let the players make content as well a la Eve Online where factions vying for control of territory IS content. This fills in the blanks between the written stuff.