What a great idea! If I were going to make one I'd think about features like:
1. A space for a water bottle (if you can do a pocket or strap so the bottle won't fall over that would be great)
2. A small ring somewhere near the top of the interior to hook the leash to (so I don't have to dig around for the hook end)
3. At least one waterproof/resistant compartment, if not the entire interior
4. A spot for plastic baggies for bio cleanup
5. A zippered compartment for treats (to prevent easy access for little noses)
6. Must be machine washable!
Well now you've got me thinking! LOL