I'd love to see a disc (as in frisbee-type) bag. I would use it for disc dog sports, but I've noticed that most disc bag options are really for disc golf. (I mention this only because the cross-application means a wider market!)
I have a couple of backpacks that I've used to haul my discs to the park with my pup, but I really want something that is comfortable to carry on my walk, easily accessible (like a sling bag), but keeps the discs flat. Water bottle and leash pockets would be great.
The bags that I have that are large enough not to bend the discs (and carry a little extra equipment) are too large. The smaller bags tend to bend the discs. Other options are more like full size duffels, which isn't at all helpful for walking to the park.
Maybe someday. Just one more thing to add to the wish list.