There's a lot of talk here about what people want to happen. I think we need to step back and think about what is most likely to happen. If we think about it from the perspective of the past, I believe we will get an accurate impression of what Cyan is most likely to do, and what the fan reaction will be.
First there was Uru Live released with Ubisoft. The promise of an on going stream of puzzles and ages attracted people from all walks of life, with one thing in common, a love of Adventure gaming.
Uru Live shuts down causing a huge disappointment in Cyan and Ubisoft. At this point the loss is mild for most, they are hurt but they can move on. For those more savy with modding and programing, begin to talk about using the Uru Live files to create their own servers. Expansion packs are released for the single player games and add new wrinkles to the efforts to create fan run servers.
Alcugs emerges. Cyan finds out about it and seem to be supporting the idea without giving any kind of permission. At that time they probably decide to release their own server software to curtail Alcugs becoming so popular they lose footing to get fans back if they find another publisher. Until uru is born.
UU explodes with creativity for the moders. Code is unlocked, studied, and rewritten. Not everyone finds this a good thing. Some in the know people hear murmurings that Cyan may find another backer for Uru soon. Many arguments break out between moders and purists. With moding comes bug fixes. UU eventually becomes the most stable version of Online Uru.
Once fairly stable, Cyan opens their own shard, Dmala. They use the fixes and some mods from the fan run servers. This is also the platform they use to sell Uru to prospective publisher. Publisher is impressed by the fan base and the game itself.
Myst Online; Uru Live begins. Cyan wants to keep their *entire* fan base. They realize that the range of moders to purists is a wide band, not two extremes. They try to find a way to please everyone and keep Gametap happy with the number of subscribers. This is a next to impossible feat.
Cyan, possibly from pressure by Game Tap, decides to shut down Until Uru. This pleases the people who lean torwards purism, but upsets the moding community. Especially since the fixes and mods that were officially allowed were removed from the new version of the game. Cyan hatches the Fan creation license idea. It had been around since Myst, but now it can be a way to keep the moders and the purists together.
MOUL ends. Everyone is upset. This is the third time the game is shut down. No one is happy, most people leave outright. Cyan attempts to keep some people on the hook with promises of putting up a server when they can generate enough money, and other things. Moders do their own thing. Many great strides in creation take place as there is no pressure to get any officialness.
And here we are. A server is up. Cyan is promising some form of open source. With nothing better to do, we argue about what that means, and how it will be run.
Now what I see is a company that wants it's fans to be as happy as possible. They want to make money off of that idea too, but they have to find a balance between those who lean towards only Cyan content and those who lean towards fan created content.
Logically it would be best to release server code again to allow fan run servers. This would allow moders a much easier way to come up with bug fixes and mods that would benefit the main hub (their server).
They don't have to release the game code. I would actually be surprised if they did. They want a healthy mod community, but they don't want to put all their cards on the table either.
They will release their tools, or tools they dumb down or rewrite, for use in creating game files. They said they were gonna do it plain and simple. Sure they could back out of this, but there's no real danger in doing this. There are already tools that could potentially do the same things that were fan made.
They will try to find some way to control what people make and put up for download. Why? It's Cyan, they like making things difficult. That's why they are the best at making puzzles. This will cause the most uproar in the mod community. No creator wants their creation put through a battery of tests unless they are in control of it. Also creators usually don't like the idea of some other fan scrutinizing their work with some BS fake fandom position.
Those are my observations anyway. I could be dead wrong. I usually am.

PS: Don't just go by what I write. There are tons of old forums to check where you can piece the story together yourself. You really can get a feel for how CYan makes business decisions.