It would be neat if there could be a Bit that does not change direction via marbles, but would easily allow a person to flip its direction while the program is running (perhaps by using a modified Bit piece that is weighted to not flip from weight of marbles, or maybe added friction, but that a user can easily switch by twisting the protruding peg). I know you can use the green Ramp as a “hard-coded” Bit, but these are meant to depict hard-coded logic, and not user-interaction. Plus, it would be a pain to constantly pull out and replace these mid-program, and they don’t really stand out as user-inputs.
This idea of user-input would open the doors to a host of new puzzles and even games by providing more ways for a user to interact with the computer by flipping various Bits while the program is running.