Dear Unity Staff,
I am writing to you to ask if you would be willing to hand me the version of Unity that was used to develop FusionFall? There’s a whole community that has reverse engineered the game, however we can only go so far, and we’ve hit a dead end.
We want to develop new content for the game client. While Grigon did go under in 2009, the actual game executives have expressed interest in breeding new life into these games via the community.
Sincerely,
Mac
You can download all prior public release of Unity here:
You would need to license them appropriately, as well as likely operate them in a hosted 32-bit environment, as MacOSX will not run them directly anymore, and most likely neither would Windows.
Do you have the Unity project for this game? Can you upgrade the project to a newer version of unity?
Well the funny thing is that when attempting to update the game at all, it mangles the client horribly. And since world building was so specific within the Unity Editor for this game, the only real way would be to have the original editor itself, which I just checked, and is not in the archive page.
Graham Dunnett did say that a staff member could go and grab the file since staff has access to that sort of stuff however.
The editor version would be
Unity Editor (2.5.5b4)
Oh wow that version is ancient. I’m not sure it would even work anymore. I’ll have to check internally. What version have you tried? Can you try the oldest one on the archive?
Unfortunately, the way world building works is kind of in an origami sort of way with pathing and sidewalks, so even if we do use another version of the editor, it wouldn’t load in sidewalk data or allow us to use the specific water material. Very broken sadly.
Why would it not load the data in a newer version? Is the project public?
There was only one version of this specific editor made. The original game was built in GameBryo, and then ported to Unity at the last minute. So to compensate and match certain details from what the Gamebryo engine could do, Unity had to make specific changes so that when porting the game from GameBryo, stuff like terrain would stay in it’s intended form.
Oh so this is a special fork of the editor? I think the chances of it still existing just went down. I’ll ask around internally.
Alright, I thank you for your efforts! I’ll check back later today or tomorrow.
Fun read though, to see people passionate about such an old game. Does seem like it’s worth chasing up a plan B to see if the community could port to a slightly more modern version of Unity.
OK, it looks like this request has been made before in 2015 
We were unable to help then. It’s a custom build that was made for a specific customer. It’s not kept in our archives and from what I can tell our source code only goes back to 4.x so its gone 
I do think you will need to update to a newer version of Unity and port the project. My understanding is that the custom code was something to do with getting the mesh/texture data out of Gamebryo format so in theory that should be possible to do now providing you have some understanding of the format or reference materials. Maybe you can convert the assets?
Why not even bring the porting out in the open on Unity forums? People love a problem to chew on, and I gather you would get help.
Thank you for the quick response. Very unfortunate news, but I truly thank you for going around and checking. We don’t know who the original order was for, whether that being for Chris Waldron, it’s unknown.
The community has taken the OBJ assets and thrown them into newer Unity versions, however because we’d want to create expanded content for the original game that was scrapped due to Grigon closing, creating new map squares, upgrading locked inventory slot amounts, etc. is just about nearly impossible without that editor.
We thank you again for checking for us, and hope to find another way to gain this level of access to creating these assets sometime in the future.
Porting something like this isn’t easy when all we have is the main.unity3d and asset bundles lol. There are a few projects for it however within the game’s community. Just search up OpenFusion and you can join in on the fun!
Ah well if you dont have the Unity project then having 2.5.5b4 would not be much use, you need both the Editor and the project.
I thought this too, however FusionFall Universe had custom tools that could take things you build in the public 2.x Unity and inject it into the existing asset bundles and force the client to load them. Towards the end of their project they did manage to actually add full new map squares with terrain, missions, and items, however their caveat would be no sidewalks, still unable to change inventory space, and lots of 2008 limitations.
If we had the project file like ToonTown recently had happen to itself a couple years ago, it’d be great. However with the Turner Broadcasting being killed off completely, and Warner merging with Discovery, they’ve thrown away a lot of their older stuff for games like this.
Again, thank you for your responses, very grateful to confirm closure here. 
I don’t mean to ruin everybody’s parade here but Cartoon Network did apparently DMCA-takedown the fan attempts at keeping this alive:
DMCA Takedown
On April 16, 2020, FFU, whose Twitter account had been changed to ‘The Universe Team’, announced that Cartoon Network had filed a DMCA takedown request, which the team agreed to, leading to FusionFall Retro being shut down and FusionFall Legacy being cancelled as it was also confirmed that the team’s fan projects, social media, and web services would no longer be hosted in any capacity. Numerous details about what FFU planned to include in Retro have since been shared by the organizations members.
Not entirely unexpected, and not worried either. Hardly hurt a soul.