Ayyy, that's a pretty cool project you have! It's really fun coming up with racetracks with this.
Some suggestions:
1. A wall obstacle. Literally just a wall that you can place with any orientation or length, to block the balls' path inbetween the grid.
2. The track nodes of bridges and regular paths being seperate, so that you can have a track and road going directly on top of one another.
3. Being able to spawn marbles / spawn checkpoints, starts and finishes on bridges, maybe via carrying over the layer button to the race mode.
4. Being able to "link" checkpoints to each other so those checkpoints all count as one big checkpoint. (useful for split paths and such)
Overall a fun thing to mess around / share tracks to friends with! 8/10 would recommend despite being unfinished