Why not just allow users to host third party servers? That would definitely alleviate the problems caused by modding, and as a plus, since people would be able to host their own servers, you could technically host a server on your own computer.
Since Origin does not require you to be online to use it, it technically also creates an offline mode without any needed extra coding done.
Also, in this scheme, the mods would be installed on the server side, beaning you are only able to access mods a server administrator approves.