A number of charities allow you to donate hair. You wouldn't think hair would be useful, but the most common use is making wigs for little girls who have cancer. The treatment for cancer costs them their natural hair, and having a wig makes them feel more...normal...about the whole thing. For this reason, they want long hair pretty much exclusively. If the hair is too long for the recipient, she can always cut it. If the hair's too short...well, not much can be done about that. If hair isn't long enough for that, it's also proven well at absorbing oil slicks. Or it can be made into brushes.
All hair use, however, is just a little insufficient. We get a lot from haircuts, and from Indian widows who are required, for religious reasons, to shave their heads when their husbands die, but we need so much more. So it's time to look into substitutes. Doll hair uses nylon fibers, and kind of resembles hair good enough for a paint brush. Not quite good enough on a human being.The texture is vaugely wrong, and not quite bouncy enough. A better substitute can be found in animal hair. Horses have some very good hair for this purpose in their manes and tails, and angora rabbit's hair would be perfect if it could be gotten long enough. Either would be fine with being shaven in hot weather. In fact, horses often prefer it, as their natural mane has a way of getting dirty and tangled, requiring vigorous brushing. Ask a parent with a toddler what their child thinks of being brushed, horses are about the same about it.
I think the best solution, however, would be reverse-engineering the way that horse and rabbit hair grows, and producing an artificial version of the same, be it chemical or biological (grown in a vat). Then we'd have all the hair we want. Wigs? One for everybody. Insulation? Now with hair for extra creepiness! Oil slick? We'll drown it in nylon hair bags!