Bethesda has (or will, it says “later today” but may already have happened) released the official Rage toolkit on Steam. That’s the same back-end that was used to create the game in the first place, so the modding potential speaks for itself.
Rage is enjoying something of a delayed lease of life; there was last year’s ‘Scorchers’ DLC, and now this mod package.
You can find hefty documentation and a warning that this kit is intended for “technically sophisticated and adventurous” modders on Bethesda’s blog page. It’s a rather fat 35 GB download, so do be sure you actually want to use it before embarking on that arduous test of bandwidth.
You’ll find the kit itself under the ‘Tools’ section in Steam.