Tires are the one thing builders can't make in the garage. I think it would be cool to see a 24x3 slik tire, that looked like the ones on old banana seat bikes. flat with no tread at all, just bigger. Everyone is making handlebars now, and you can get em made to your design even. I don't know how much call there is for expensive new forks. I think some of the new wide triple trees they make for offroad fat bikes could be cool for modification. Somebody is making a really wide monark looking springer for fat tires, but the price is really up there. Leaf spring forks are cool, maybe kits to make them instead of selling them all together. People still love doing as much as they can themselves. I think good inner tubes are hard to get! And nice fenders are hard to find as well.