I have done a lot of this type of work before.
The first real project I ever attempted to do was a total conversion for Quake 3 called Gundam Quake 3. I had attempted to be project lead, and even sourced a modeller to start with. Full design of gameplay was done, and the initial testing was started. Ran into some bug issues such as models "exploding" during gameplay, but it never really got very far because everyone disappeared and I ran into some technical roadblocks that I couldn't solve.
Ref: all posts were on PlanetQuake and other forums that are no longer online
Images: none at this time, will see about posting something later.
My second project was to try a similar thing with Doom 3, except just do it in my spare time and not as a full blown project. It was an idea to have a mystery type story based on G.I.Joe. I did not get far past doing a general write up and nothing past doing some texture replacements. I do not really remember why I didn't continue on this one. T
Ref: all posts were on PlanetDoom which no longer exists.
Images:
I had also done a similar attempt to try to do a conversion of sorts for Diablo II. I had done the usual story and layout drafts, and also had been able to rename NPCs, change dialogue, create items and such... but I think the problem with this is that I did not have the skills to properly create the character animations.
Ref: all posts were on PlanetDiablo which no longer exists.
Images: none at this time
Somewhere in there I had also wanted to create Quake 3 maps based on scenes from Transformers: The Movie. This was an interesting thought but I ended up quitting due to the quality of the mapping tools available at the time and the inability to handle the complexity of the maps I wanted to make. If you haven't built a Quake 3 map before, they need to be "airtight." The design of the map is visualized best if you think of your canvas as being a block of water. You create the rooms/viewable areas inside of this water and it must not have any holes. Having a hole, even if 1 pixel large, would cause an error! And it was frustrating to fix these holes or seam issues because the tools would not always tell you where the problem was.
Ref: unknown
Images:
Note: the gun and head model in these screenshots are my normal model.
And then the last one I had worked on an had some success was NHL06. I was NHLDepot.net's official roster maker for NHL06, which involved keeping all the NHL team rosters up to date, in addition to Eliteserien, SM-Liiga and DEL. Also worked with other talented modders to create a "package" of mods required in order to make NHL06 up to date and reflect the actual game. This included updated jerseys, real life ice surfaces and boards, presentation graphics, logos, all that stuff. A result would be something like this (note: NHL06 did not have Anaheim Ducks and the Buffalo Sabres still had the black jerseys)
I ended up stopping this project because of the amount of time it took away from me. Having to monitor transactions for the NHL and the three other leagues mentioned above on a daily basis was a half an hour to 3 hours (or more for big days like draft, free agency opening period or trade deadline) and could not be skipped. Taking a day off from checking the sites would mean having to do even more work the next day, because real life does not stop just because you were tired. And it also involved reading the transaction logs for various websites, including ones in Europe where (at the time) this information was not always updated or accurate. Having to navigate websites from Finland, Sweden, Germany without knowing how to read the language and needing to cross-reference information when you aren't sure about something. Doing up-to-date rosters for a video game may actually be a full time job! And another issue is that when you take on a task like this, you end up not being able to play the game. I had started that project because I loved to play the game. But then you do all this work and you do have to launch the game to verify the files are good, but you don't have time to play, especially since the files are changing daily, you can't play a season or something because the data will change before you get to play again.
Ref: most posts on NHLDepot.net, site is still around but forums are gone.
Image of one of the editing interfaces (the packer tool)