Not sure what people would think of this, but having an Advanced tank design with the following features. (This could help cut down on total number of parts as well)
Hold XX Tons of fuels. (adjustable with a slider of fill %, IE: you could launch an empty tank like you can now) This would be useful for space planes since the methane usage varies.
Add chamber(s) of propellant up to XX% of methane, oxidizer, monopropellant, or of hydrogen, but not xenon. to make this more user friendly, you could default it to methalox settings with 2 chambers of the X * Y ratio.
Each additional chamber adds 0.xx T of weight for balancing.
The tank would be deeper in the science tree to avoid it spoiling the fun of early rocket shenanigans.