I can see a stepwise transition from a single player RPG to a game which for all practical intents and purposes is an MMO.
Step 1: Start with a reasonably fun and popular single player RPG.
Step 2: Add DLC content that requires multiple players and teamwork to accomplish an objective. Say, a boss too powerful to be solo'd, with trash mobs too numerous for one person. Being of greater difficulty, one will be rewarded with better loot. Initially, for a 10 person instance say, one could que up alone or with friends and once 10 people have queued the instance will begin.
Step 3: Create a lobby server that can accomodate 100 players, or so. The more the better, but not so many that performance suffers too much. If 300 players are on you can have 3 such "hub" instances, with a LFG channel that spans all instances. Rather than from a menu screen, you can put raiding parties together from in-game. Plus you can chat, and roleplay, and emote, and what not while waiting for your group.
Step 4: Add some guild functions, and at this point you are most of the way to an MMO.
Step 5: For regular single player questing areas, let there be optional servers to log into where other people might questing at the same time.
Step 6: Throw in a couple of PvP battleground instances where you can sell your victories and honor kills for better-than-average loot.
With these steps, you have a basic MMO that doesn't require that much more development than what routinely exists for single player games with multiplayer content. Except where multiplayer is usually competitive, here it would mostly be cooperative PvE with people coming in an leaving all the time.