Separate bipropellant into their individual propellants and allow bipropellant fuel tanks

Hydrolox will become liquid hydrogen and liquid oxygen. Kerolox will become liquid kerosene and liquid oxygen. Methalox will become liquid methane and liquid oxygen.
Fuel tanks selected to hold hydrolox/kerolox/methalox will automatically calculate how much of each fuel type to store (mixture ratio is not accessible to the user).
Rocket engines will be able to consume an arbitrary number of arbitrary fuels (or produce them).