I have a few things to add here.
First is about the crested. A few breeders have found that you can selectively breed for crests that are NOT lethal gened. Metzers, for example, breeds and sells crested layers. These do not usually have the same issue talked about above as they don't all have the vaulted skull. So with selective breeding, you can breed crested to crested (this is the best way to get the large, puffy crests) without the issues.
Second, feathered legs. I breed anconas and always get a few each season that hatch with feathered legs. They tend to fall off with age though. It takes very selective breeding to get ducks that keep the feathers into adulthood. Check out Snyder ducks. They are a feathered leg breed created by a fellow in Canada. He does a fantastic job and has been breeding them for many years now. Aside from these and anconas, I have yet to come across another breed that has feathered legs. Though I'm sure they are out there, it's just not documented!
Third, the mini-muscovy. You can breed smaller framed muscovy. However, any that I have seen or read about online that are much smaller than usual tend to be birds that fail to thrive hence why they are stunted. These should not be bred and tend to have health problems along with their smaller size.