If it were an MMO, would they go the grand environment route, everyone for themselves, the classic MMO stance like WoW, Lineage 2, etc... or the instanced route with lobbies, Raid Parties and such like with dungeon runs and Phantasy Star Online/Universe? It could so easily be balls'd up though, I mean seriously, how many MMO's become successful? So few that you can count them.
Game gets released, high praise, couple patches get released to fix major bugs and add content. Then the major lull happens and the 'Balance' patches start. Player skills/weapons get nerfed, items removed, they remove farming locations by changing spawn rates, the influx of spammers begins, GMs too 'busy' to ban the spammers on site. Next patch, more balance, since that last patch made certain skills underpowered, this now makes other skills too powerful, so they get nerfed... which makes other skills too powerful, so they get nerfed, and repeat and repeat. Then the big expansion comes, with more skills, new locations and more balancing... by this time 70% of the original players have moved on. The game looses direction, so they ask for feedback, the direction of the game is now handed over to a small vocal minority that play the game 27 hours a day, and thus concentrate all their time on PvP pwnage...
The above has repeated itself so many times over the years i've played MMOs and it's quite sad when it does. When I see people wanting an MMO of an RPG, they're just setting themselves up for disappointment. Borderlands is fine as it is, it's a multiplayer game that doesn't really need an MMO element, the system works. They could make a few changes, bits of extra content and such. Maybe introduce a closed network system like an MMO to prevent game save hacks. People will still call it an MMO, or an MMO with an identity crisis, but the underlining mechanics are fine... it's become successful for what it is, so why change it?