A complete redesign of the modules to add a power line transmitter to them.
X10 at one time had a few two way modules. LM14A Lamp and AM14A Appliance. Now discontinued.
There are two way modules made by other manufacturers, but a a much higher cost than X10's.
One Smarthome Insteon wall switch that can have a X10 address added to it is $46.00. They handle bright and dim commands differently; when locally set with their local paddle; sending data back on the power line for the controller to receive. Their Icon version slightly less in price.