I keep meaning to DIY some stuff myself, but never get around to it. Here are some things I've thought about though, in case it helps. I'd totally be into patterns.
Shoe covers- not necessarily über-waterproof ones, just something that goes over the front of my sneakers to keep them from getting soaked. Was thinking duct tape for this one, maybe
Panniers- waterproof (covered zipper, seamgrip for sealing seams). Would like them to have the side closest to pedals tapered back for better heel clearance, also would like them to be of cuter fabric (thus, the DIY)
Pants- I'm sure a standard pants pattern exists that would work with waterproof fabric, though. I'm having trouble finding any that fit (rise too high, length too long).