As far as I know, MPBT 3025 was the game that came closest to an MMO.
I think we can come pretty close to an MMO experience if we have something like a well-organized MW:LL league on this board.
What a battletech MMO would need to have would be a market or store, a place for players to meet and "missions" with winners and losers that influence a storyline. --- in a very rudimentary way this can be realised within this forum.
You join the 'mmo' board, choose a faction. You get a rank within that faction. Your profile contains your faction, rank, the mechs you're allowed to pilot, your posessions and played / scheduled missions. Those profiles are saved in a forum thread.
All MMO players would take part in a shared story. Whoever tells this story is responsible to create missions. Missions are designed and planned on the board and MMO players can enlist on these missions; the missions would be played on a private server, where an admin gives the players money acoording to their MMO rank so they can only buy the mechs that they posses on the board.
When the mission is over, the info from the scoreboard a is added to the MMO-players profile. If you have earned money you could use it for example to buy new "mech licences" that allow you to drive other mechs or mech variants.
The mission outcome will affect how the story of the MMO develops.
It's cheap, it's nerdy. But it doesn't take a new game that has to be programmed forst; all it takes is some commitment/dedication. When I was younger I was on some forums where they created Battlefield 1942 scenarios that way, and it worked.