Unifying all engines (jet, ion, rcn and rocket) into the props.xml can help a lot to make custom engine types, from different fuels for jets and ion thrusters to hybrid engines or electric consumption/generation.
Just adding air and electricity consumptions (with negative values available in the electricity consumption attribute) can be a great first step, relatively easy to implement while opening a door for future addition of the jet, ion and rcn to have a fuel selector.