I think it'd be really interesting if one had the ability to tweak different elements and compounds in planets. Think things like Ammonia oceans or Carbon planets. Adding to this, it would be interesting to manipulate atmospheric pressure to get things like warm ammonia seas or seas of carbon dioxide. Xenobiosphere simulations commensurate to these environments would be truly awesome.
Also, with simulated life, one has to take into account the light of a planet's parent star in depicting plant colours. A yellow-white star like Sol has green plants, but a red dwarf might have blue or violet plants and a blue-white star might have orange or red plants. So many possibilities.