I wanted to combine a few of the features mentioned above.
I am thinking 2-3 pieces.
the first is a neoprene base with 1 or 2 zip pockets. 1 has a space for earphones.
this neoprene pocket can wrap around a waterbottle or the arm of the treadmill.

Part 2 is a clip that you can attach your standard gym towel too - that way you don't have to bring your own towel and the gymcarry can use any towel from any gym. I like the idea of a towel with a pocket but I don't really want to bring my own towel to the gym and back home when there are clean towels at the gym.