Create Multiplayer only game and make it Moddable for users?

I was wondering if I could create a game in UDK that could be Multiplayer only and have it so other users can create custom content (Bipeds, Scenery, Level BSP’s, Weapons, Vehicles etc) and make it into a map for this game so users can create there own levels with a lot of possibilities. So users would be able to create single player, multiplayer, and co-op maps. (Can UDK do Co-Op?)

Thanks in advance, Tyler.

This quastion fits to both UDK and UE4.

It depends on game design all data needs to be in both server and client. So if you let user host the server, then yes. If user can create content in game and server other clients can be quicly synced then yes, but code need to be set in way that user content could be replicated. If either of those and you gonna host a server and content wont be created by game it self, then no that would require you to add submitted user files to server.