Star Wars Battlefront 3 Legacy is a total conversion mod for the 2005 release of Battlefront 2. It uses assets from the cancelled Free Radical sequel to imagine what might have been if the project had shipped. A pre-demo is available now on Mod-DB (opens in new tab).
According to the mod's creator El_Fabricio, "this version comes only with Battlefront 3's Coruscant, the Clone Wars era, three gamemodes and a lot of updated and new content from the previous beta versions." You need to install the Unofficial patch 1.3 contained within the mod files, and the [GT]Anakin's HD Remaster (opens in new tab).
It's still a work-in-progress. "Map reworks, new maps and the GCW era are planned to be released within the coming months" and "new heroes for both eras will also be added with each bigger update".
For more information and full installation instructions check out the description section of the ModDB page (opens in new tab). We'll never get to play the exact game that Free Radical Design were developing, but a few years ago some footage emerged of the first level. Enjoy that nostalgia hit below.