I just published a new game with the same driving that I plan on updating with time. I don't know anything about modding but I would love to include a track builder.
I have done a decent amount of research, but from what I've seen, it appears the only way modding can work with a Game Maker game can be modded is by posting source code. I do totally understand if that's not a viable option for you. I just had a few stray ideas on how to make the game a little different.