Paniers, sorta:
http://www.backpacking.net/makegear.html
My fenders in bamboo
I disagree on the R&D, it is pretty trivial, at least if one incorporates the bulk of it by just copying the main information. There are also some rather bad designs that just never go away, presumably because lots of people like them.
The first place to make one's own adjustements is by serving oneself. For instance on the touring bike frame I am building, I have designed it to take three brakes. I like this idea, but no time that I have mentioned it has anyone jumped in and said "cool!'. Lots of people seem to really like the idea of top moutned pass-through levers. My approach is way better in terms of stopping power, modulation, redundancy, parts availability, fork loads, everything but a little weight. I copied it from a tandem rig, so I can't take credit. It doesn't bother me nobody likes it, it just suits my particular circumstances.
Another example would be the wild redundancy in racks and paniers in order to accomodate quick release of the paniers. I never remove my paniers from the bike, so quick release is not important to me and I can think of a way of doing it anyway without the frames hooks etc... Build your own paniers without the built in frames and you save a ton of weight. No particular R&D required all the fititngs and design features are there for anyone to copy from external frame packs. Obviously a lot of people prize removeable paniers. My point is that making effective modifications in one's own gear is perfectly possible.
Another example is I want to build a frame that has Tandem like strength in it's components. You might think messing around with that sort of thing would require a lot of engineering. But between studying specifications, asking for tubing recomendations from the companies that supply tubes to your favorite builders, and examining various tubes, there are only so many actual options and they sort themselves out pretty quickly.