Modding is a hobby that players of single player computer games do to redesign the way their game works.
It's only single player games or non-massively multiplayer games that currently support this.
There are emulators of commercial MMOs that exist. They are not quite the same thing. First they are really hard to make - you have to reverse engineer the client then write code for your server to handle the jobs it needs to do then get it uploaded and running. They suffer from being not legal and highly vulnerable to cease and desist orders. These are not what I am talking about in this post although hats off to the pioneers who do them.
What I'm talking about is software that is designed to let amateurs build mmos. Possibly commercial software, possibly game engines of old mmos that are being shut down.
With MMOs there are several barriers to modding.
1) You don't have the source code. Unlike a game like Oblivion where you have the whole game in a MMO you only have a small part of it, the client.
2) Business models depend upon not letting people set up free servers.
3) People are bastards. If you set up an amateur MMO that was completely free people would turn up to try to hack it, crash it, grief players etc. This is just the nature of the Wild Wild Web.
4) They are incredibly labour-intensive to build.
I believe that over time the first two problems will inevitably collapse. This is because of Stabs' Law. No matter how pointless or bad for the gaming industry it may seem to be at some point someone who owns the rights to an old mmo will stand up and tell the world, thanks for playing, it's been a great ten years, we're shutting down the servers, here have the source code, it's uploaded and open source. Just because. Stabs' Law.
3) is a problem that is solved over time. Suppose knitting circles didn't exist and someone just now came up with the idea. There would be warnings that knitters could get mugged while travelling to the circle or that one of the members might be an axe murderer. People might turn up to them just because they are new with no motive except to be disruptive. Eventually though if people keep going along and knitting the disruption just falls by the way side, it's no longer interesting to do it.
If there are, one day, thousands of free amateur mmos out there it simpy won't be exciting to travel around griefing them.
4) is the reason I'm investigating mmo modding and beginning to communicate about it. I want to tell people what might be possible, how I'd like it to work so I don't have to do elements of mmo-building I don't like and/or can't do (for example coding or designing combat animations). We will see amateur mmos, they're going to be occasionally brilliant, they're going to become increasingly fun to make over time and they will be worlds that if you are reading this you will probably play in or create before the end of your lifetime.
I look forward to seeing some of you in Querulous Plains in about fifteen years' time!