2 dollar bills
I have been told that the main reason why $2 bills and 50-cent coins are rarely seen in the U.S., although plenty were issued, is because there is no slot in the drawer of a cash register for them, so retailers discouraged them and didn't usually give them in change, since they tended to get thrown into the bottom of the cash drawer, under the tray with the slots holding the other cash. This is, I think, the main reason why $1 coins do not catch on. I doubt very much that $1 coins or $2 bills will ever catch on until the U.S. govt. takes the very sensible step of discontinuing the $1 bill, which has been discussed for a long time.