The problem is that his question is too broad and open-ended. It’s like if I went on an engineering forum and asked “how do I build a space rocket?”. There are so many ways it can be built and designed, depending on what you want to do, this thread could spawn 1000 answers debating design approaches.
Also, if the OP is asking a question like that, it shows they have no knowledge of UE4 or game design, otherwise they would be asking more specific questions.
To the OP: the most constructive answer I can give you is to put the MMO idea on hold and first learn how UE4 works. Try making some smaller, simpler games to learn how to use UE4 first and then tackle the MMO when you’re experienced enough to design it yourself.