Yep, combo'ing user maps (and some of the other map types already supported) would be very cool.
We are planning to move to a newer directX version which should allow us to do a lot more cool things that graphics cards are already good at doing quickly. Blending, streching, rotation and the like...
It's going to change the way a lot of TF works (for the better) and hopefully combo will be both faster and more versatile.
Thanks for the idea.