One issue is that when utilizing (non-Node) JS there just isn't a good way of forming TCP connections or sending information over UDP. A server has to be used as an intermediate. RPG Maker MV supports two modes, a web mode that utilizes vanilla JS and a desktop mode that utilizes Node.
This means that for the RMMVN project to be completed there could be a number of separate implementations depending on how the game is intended to be deployed. I'll assume that the player base for an online project in the engine will be rather small...
A http client server approach would work for both the web and desktop versions
But running a game through a http server can be very slow unless the information passed is very small, in order to get very good performance from client server we need to connect directly to a server and keep the connection.
Starting with this approach only Desktop mode is possible. Connecting directly with TCP is useful for state-sensitive data and UDP is good for data that isn't state-sensitive.
Client Servers may be a bit cost prohibitive for the common RPG Maker user though, so we can utilize Node to allow for P2P to ease the pain.
The server could just be used to route connections between players through easy to use HTTP.
In this example each player makes one request to the server and 'logs in' which is recorded in the database. A player could send a (extremely lightweight) signal to let the server know they're still 'logged in.' After a certain amount of time they would be removed the the database. The server would then send a list of ports and IPs to allow the local Desktop game to form TCP connections between the players(bad connections are simply rejected to simplify things since there is no way to be sure that a received IP is logged in.)
The very last approach that could be implemented is a direct IP approach where you manually enter the port/ip in-game to form the connection(no server necessary.)
It may be awhile before I get to it, but I think if I were to get back to this project I would try to implement easy to use solutions for all of these approaches. If someone wants to do this ahead of me then that's fine too. For those willing to attempt it I have created a NoSQL Database usable within the Desktop Game application which would be useful for P2P.