I think it would be interesting (if there is anyone out there who knows how to mod golfing over it) to have custom maps and possibly make them into speedrunning categories

I honestly don't see a need for more categories. We already have multiple categories with less than 10 submissions. In my opinion, the leaderboard would need to be much more active before we consider adding extra categories. However, a custom map system would be interesting to see. Not a bad idea.

Getting over it has an active modding scene. I know absolutely nothing about modding though, so I wont be of much help there.

Custom maps on leaderboards cause tons of issues across the board whenever they've been allowed in other games. It's very unlikely that we allow them here. You could try to request a board for a specific pack or something though and make boards for that there.