I decided to see if it were possible to create a Yume Nikki MMO. I noticed various issues while creating it which may help others when making their own.
* Scalability: the rooms in Yume Nikki are small so fitting a lot of people in one room would require servers or duplicate rooms. This mainly happens in Madotsuki's room where only about 40~60 people can fit (unless you allow players walk on top of each other). You can solve this by making custom rooms or by isolating players.
* Isolation: Yume Nikki as a game evokes the feeling of isolation throughout the gameplay. This mood will obviously be destroyed if it becomes a MMO. This again could be solved by making specific rooms only for one player. By this I mean the room can be accessed by multiple users but the user will only see themselves in the room and not the other users.
* Identification: I initially intended on creating a chat system for other users to talk but then I realized that a method for identifying other users would require some kind of Identification. Like a name tag or something below their character. To create something like this you would either have to make a tiny font which would be barely legible. Or break the pixel rule and put an actual font in. This often breaks the aesthetics, here's an example: https://i.imgur.com/sjeJH1S.png
While other parts like making all the rooms and events work properly are difficult they are difficult in the sense that they are a tedious task rather than a technical difficulty. These were all the issues I could see.