First thing I'd like to ask you, is whether you'd want to change the file name ("race" sounds so...unpersonal).
And this is not a question, but a demand: what did Super-Sheep do exactly? You based off one of his maps, which one? Did you get permission etc. Best thing to do is link the map you edited it from and add some sign of his approval.
Here are some other suggestions:
Cliffs/hills:
It'd be good to see some more variation in the terrain height.
And then I don't just mean using a tool to increase/decrease the terrain height, but also let it influence the speed of the runner.
The steeper the cliff, the slower you go (and vice versa).
Tracks:
Create more tracks. Definitely.
They sure don't have to be as long as this one: very short tracks, where agility and a good hand-eye coordination are the most important factors of winning the race can greatly increase the variety of the game.
EroticSideburns basically said most about the tracks. Create a good (better than this one) terrain, various terrains (perhaps with different effects too: dungeons might contain earthquakes, which shake the camera).
You could also place a few top-down camera's and make the host (or the first player) be able to select a map (with arrow keys would be the best choose system).
Misc:
When you finish the race, there's not much to do.
You just wait and check the minimap, hoping the opponent is almost at the end.
When he IS at the end, nothing happens (at least, I didn't see anything happen). Perhaps a restart-option? Or at least a little sign that tells you the position of the runners and in which time they completed the track.
Something to make you think "Oh, everyone's finished!" instead of "Player X finished".
I know your description already said something about abilities, and you do indeed need them.
The problem is that I don't really have a lot of inspiration.
I can think of "Jump" (which would require you to track whether the jumping unit is actually jumping OVER the object, or against it - not the easiest to code) and maybe somehing to let you resist slowing down in grass (no name-idea here, but easy to code and implement).
Another harder thing to code: slipping.
If you don't use 1 speed, but 2 speeds (speed at the X-axis and speed at the Y-axis), you can create a slipping effect. This slipping effect can be bigger on wet/slippery surfaces (The farmland for example).
When done correctly, it adds another difficulty factor to the game.
Yet again, EroticSideburns came up with some other great ideas as well. Different racers, although not required, are a nice add-on all equally powerful, checkpoints! Another one to definitely put on your to-do list.
Good luck.