I cannot fathom how you could turn a side-scrolling game into an MMO game.
Remember that MMO has a meaning. It was a term for games with an enormous online population, these days meaning about 100,000 concurrent people.
While a lot of kids playing with their toy systems will call any game that plays online an MMO, that is not what the term was referring to. Richard Garriott used the term to refer to the type of architecture required for the modern sprawling high-player games with hundreds of thousands of active players. These systems are far beyond a dedicated system, far beyond something a moderately large team can generate. They are truly a massive undertaking, costing several tens of millions of dollars annually in infrastructure alone. The infrastructure and day-to-day operations costs more than the development cost for most games.
I know many youth these days equate "MMO" with "persistent online world", but they are radically different. People can (and do) build a persistent online world infrastructure in a few hours. The networking forum FAQ has one written and and playable in under a week. Persistent online world, but a maximum player capacity of perhaps a hundred or two simultaneous players.
These days they have a notation of the number of concurrent players. Five thousand concurrent players, a C5K system is achievable with simple architecture and isn't massively multiplayer. Ten thousand (C10K) or fifty thousand (C50K) is achievable on a budget of ten or twenty million dollars, and you could probably host that with Amazon or Google or similar architecture that scales dynamically, you could reasonably handle that with a cluster of 10 or 20 servers pretty handily. But even that isn't massively multiplayer, that's still just a regular online world. When you reach larger scales, C100K, C250K, that is reaching a massive architecture. A quarter million concurrent players means an enormous infrastructure spread across the globe.
I cannot fathom tens of thousands of people, or even thousands of people, in a side scroller simultaneously.
I can imagine such a system with hundreds of simultaneous people, but that is painful in my mind
Maybe you've got a design that is different from the side scroller style I picture.
So if you want to build an online multiplayer side scroller world, that certainly is achievable. But I cannot fathom an MMO side scroller no matter how I try. The "massive" modifier is a big thing.