Think big. Start small. Struggle. Persevere.
Start developing it. You’ll understand what you need for your particular game soon. How small should you start? Create a server capable of handling data for a global instance that governs: players each positioning their own sphere character and players setting the color of their own sphere, which everyone sees. Then expand. Expand into data for players changing the color of other spheres via shooting; regarding and disregarding important or superficial forms of data within a range; knowing when others are jumping; and so forth. The heart of every server matter like this, regardless of whether it’s a real-time game or a real-time site, is bandwidth, speed, and thinking about where milliseconds matter most and where they matter less. That depends entirely on the needs of the experience you want and what you can afford. You can have both a large seamless world and separate servers at the same time. That depends on how data should be divided and what a user must know at all/relative times.
You’re more likely to get to the point where networking is the easiest part. Retaining players who want to play your MMO is the hard part. That’s about the game itself, the business model, staying secure & resilient, and standing out in a crowd. This holds true for all games. What is Warriors? Got a site?