Actually, one can build a MMO with the Hero and Lumberyard Engines. Its a matter of content and lots of it. To maximize the use of assets, I’d recommend all Entities be constructed from modular interchangeable parts to create many different variations. With Materials, a myriad of variation is achievable. After all some form Character customization is needed, might as well reuse customization systems for other entities.
However, If you’re going to pursue such and endeavor with UE4, I’d recommend getting the Online Subsystem MatchMaking for networking over the internet working first then expand on it. Servers can use the same matching to find other Servers to implement a form of Sharding, Zoning, Instancing, and Cloning. With multiplayer operating over the internet upfront you can build and stress test multiplayer properly.
I would not jump into game mechanics firstly. In fact, I would build a multiplayer in-game level building system. That way multiple developers can build the massive world collaboratively. This Multi-user Builder can be the template for other Multi-user Entity Customization Systems for Characters, Weapons, etc.