I love the O2/H2 dynamic, but I would love to see more done with atmosphere. First, I'd like to see more done with atmospheric composition. I'd love to see N2, CO2, and miscellaneous volatiles that can be harvested directly from the atmosphere and processed. On Mars, for example, the atmosphere is made primarily of CO2; in real life, that CO2 can be converted into O2 and used for fuel and breathable O2. On Titan, the atmosphere contains a lot of N2 and methane/ethane; these could be processed to make H2, fertilizer, and a variety of other carbon and hydrocarbon products.
Second, I'd love to see more done with atmospheric pressure and partial pressure: A planet or ship with 1 bar of atmosphere may not be breathable if it has no O2, while one with 0.2 bar would be breathable if it was 100% O2. Additionally, a high-enough pressure might be fatal to engineers. Perhaps there could be special high-pressure-environment suits for Venus-like planets. Maintaining a higher-pressure internal atmosphere on a ship might also take more energy in a low-pressure or vacuum environment, and vice versa in a high-pressure environment, so you might want to tweak your ship or base atmosphere settings to be optimized for its environment.
Third (and this is less of a priority to me), it would be cool to have some temperature-based functionality: planets that are too cold or too hot might drain your suit energy faster, or make your vents/cockpits use more power; and when your power is depleted, your character takes damage, similar to when the helmet is off in low- or no-O2 environments.