First, just a heads up: The forum for GutterTrash is usually the best place for this kind of thing. In the future, feel free to ask in the main thread, or start your own. If you DID and I somehow didn't see, my apologies.
Generally, fanwork of all kinds is fine and encouraged with two caveats: 1. No profit 2. Clear up-front identification as an "Unofficial Fan work". IE, making it clear I'm not involved in development, writing, approving things etc.
If you're planning to integrate it into the existing game that complicates things a little. If it's fully stand-alone, a mini game of its own, great. If it's a mod that can be DISTRIBUTED on its own and installed on top of a GT installation, the way translations are, also fine. Anything deeper integrated than that is probably a no-go for now, as I don't have time or energy to work on side projects at the moment.
But as long as you're content to do your own thing, and again make it clear it's unofficial, non-canon, sure, knock yourself out. If you end up doing something with it that's HORRENDOUSLY brand-damaging we may need to talk, but I don't anticipate that being an issue. Would be neat to see what you come up with.