I would love to see diverse propellant types. Hybrid rockets, tripropellant motors (Such as a Hydrogen-Flourine-Lithium system), antimatter-matter annihilation drives, hypergolic motors, and/or a variety of traditional bipropellant systems (With fuels like Kerosene, Methane, and Liquid Hydrogen). These different kinds of chemical propulsion could be augmented by different resources and resource gathering technologies for every interplanetary destination. Separate systems must be used to extract Methane from Duna’s atmosphere as are needed to electrolyze water (Hydrogen and Oxygen) on Laythe, for example. If cost factors into decisions made during any sort of “career mode,” a hypergolic rocket may be used for simplicity’s sake rather than a more efficient bipropellant rocket. Or if efficiency is to be maximized, the all liquid Fluorine/Hydrogen/Lithium system (with it’s tested specific impulse of 542 seconds!) may be the best choice.
