Add a redstone component called a Logic Gate. It has two inputs, one output, and an internal 4-entry table that specifies for each combination of inputs whether the output power level is 0 or 15 on the next redstone tick. Inputs for this purpose are identified as 0 (not powered) or 1 (powered). The table entries are programmable using a UI panel. The processing for this component is trivial and would not impose a burden on the redstone processing loop.
Obviously, this component serves as a programmable logic gate capable of implementing an arbitrary Boolean function on two inputs with a single block. For instance, it could be configured as an AND gate, OR gate, or XOR gate via programming, with no additional components necessary. This would enable existing circuits to be built smaller, faster, and more cheaply, or allow additional features to be added in the same amount of space.
One thing I haven't worked out for the Logic Gate is where its inputs are positioned. They could be on the sides for symmetry, or on one side and the back for topological convenience, but the latter would imply an asymmetry that's foreign to redstone. Maybe it should be the back and either side like a comparator, with the side inputs implicitly ORed internally.