Fuel Customization
It would be cool to choose what kind of fuel your engines use—water, xenon, methane, hydrogen, kerosene, RP-1, etc., each with its own benefits.
Maybe add the option to decide if you want an oxidizer—things like oxygen, hydrogen peroxide, chlorine, fluorine, or even none at all, which would affect how the engine burns fuel and its overall performance.
It’d be great to have a way to adjust the fuel-to-oxidizer ratio, so you can fine-tune the engine’s power or efficiency depending on what you need.
Also, a fuel-oil ratio for engines that require lubrication would help balance longevity and performance.
All of this could be done through sliders or inputs in the Part Variables menu, giving a lot of flexibility.