A federal judge recently ruled that the current paper currency is unfair to blind people because they have no way to tell the bills apart. Which seems like a good point. I've heard that blind people devise systems to tell bills in their pockets apart, such as using different pockets, folding corners, etc. But I guess it would be a problem when getting change from a purchase.
That said, the suggested proposals of embossing, punched holes, or different sized bills all seem to have problems. Embossing could wear off, punching holes seems to be inviting tears, and changing the size of the bills would probably break all those existing bill acceptors in vending machines, parking garages and so on. Of course, I've got no better ideas, so hopefully the Treasury has some smart guys on their payroll to figure this out.
So I guess we'll probably have yet another new kind of bill in the not-too-distant future, and the Treasury hasn't even gotten done releasing the previous set of new bills.