It's a big challenge and big risk to make a single-player game to MMORPG. First of all it needs a class system. Fortunately previous Borderlands games have a vague class system. For instance, Roland in the original Borderlands is a "Soldier" and Lilith is "Siren" class. In Borderlands Online, 4 classes are available: Solider, Hunter, Siren and Berserker.