Different fuel types such as methane, hydrogen, RP-1,
liquid oxygen (obviously), and other various fuels. The player would then have to design their rockets with appropriate fuel types depending on the engines they use.
Jangus Wright commented
maybe a version where you can choose maybe just Kerosene/LOX or Liquid Hydrogen/LOX, solid fuel mixtures or hyperbolics (for upper stages) and Hydrogen Peroxide Monoprop for each of your engines. maybe RP-1 is the cheaper but less efficient option, and hydrogen is much more efficient but more expensive. the iSP of each engine can change based on your fuel (and maybe oxidiser) choices.
Also factories to produce it on the earth