To run MODS in a MMO you would have to create your own server, with your own basic world, somehow create mods in a game engine that you don't know and you end up with work for a whole lot of basically the same game, but with some random add ons, praying to not only have people join, but also pray that these players help u finance said server
I could see there being sound mods that change or "improve" the game sounds to fit a player's desires, maybe they want the lasers to sound like star wars blasters? As long as the mods don't give any advantages (so only audio/visual) then I could see there being a small market for it.
I'm on the fence about mods in MMOs.. Seeing as SB involves PvP and hard losses of possessions, I am content with no mods/workshop content... Especially with features like coding in the game and it having a huge influence on being able to gain an advantage in a fight... I could see that being a game-breaker for sure with mods assisting in coding or preset codes written.