I spent almost two years on this exact project. Ultimately what we decided on was a goretex "dry bag" with a zipper sewn in a V shape on the outside of the leg. The bottom of the bag was big enough for a size 16 boot, and stayed same diameter all the way up. Slip over boot and pull up leg. connect zipper and zip up side of your leg which pulled the fabric tight enough so that it form fit to your leg. Shock cord drawstring that tightened top, then fastened to a belt loop. I had neoprene overboots called "cleanboot" I think that I sewed on straps to cinch tight around my ankle and over top of boot. Slipped bottom of bag and boot into this and locked it all down.
It worked great in the following respects. It weight 1lb for each pair, and fit in a backpack all rolled up. The bags could be used as game bags, or as dry ish bags for storing layers, camping gear, etc. They didn't require you to take boots on and off. easy to clean and dry out.
It didn't work great in the following respects. It sucks to get muddy wet bags off of you when its really cold, or really muddy and you have no way to clean your hands. No grip on the boots, though a rework of the overboots with a sole and grip of some sort would be fine.
Ultimately they were just too much of a hassle. I bought a pair of chota hippies and simms lightest weight wading boot on a last ditch effort, and they are awesome for the way I use them. Here's the catch though - most of my hunting is done above freezing. I don't mind having a wet boot while walking - it doesn't bother me at all. While sitting still for whitetail hunting, I have a pair of boot covers I sewed pockets for handwarmers in. I can get down to freezing with them wet, and well below with them dry.
The hippies are awesome for all day wear when you don't know when you'll encounter water, but know you will. Just roll them down and up as needed. I put about 60-70 miles on them this year and the system works great for swampy marshy hunting. I walk a lot and sit little, so this was a pleasant surprise.
If I were using strictly for stream crossing, the system I came up with above works fantastic. Especially where the bags can pull double duty, and extra especially if you're in a place with more rocks than mud and don't have to deal with dirty hands. I don't think this system would work good if you need more than hip high, as you wouldn't be able to use seperate "bags" and the packability breaks down.
The concept is really simple, and we kept it in house in an attempt to bring something to market. Unfortunately, I just don't have time, nor the creative juices right now to get it next level and make it worthy. If someone wants to run with it shoot me a PM I can get you in where I left off with it.