So heres' an awesome idea! Planetary Biomes define what the core of the planet is like! Most 'normal' planets, like Jungles, Forests, Deserts, would all have the same lava core, although perhaps with different coloured lava to set them apart and make them more interesting. Tendril planets, not-yet-introduced 'Apocalypse' planets, Ice planets and the no doubt crazy ones to be introduced should all have relatively custom cores. Tentacle Planets should have appropriately writhing cores, filled with living limbs and pools of odd purple/red juice. The further you go, the more organic they seem, until you reach a sea of blood, filled with active and angry tentacle beasts. Ice planets could have massive icy cores, with it's rarest minerals left frozen in the ice. So cold, even the best armor may require some fuzzy improvements to bare the chill. Perhaps howling ice-beasts roam the caves? Apocalypse planets, if ever introduced, should have sewage filled tunnels and massive sewer hubways as cores, filled with zombies or something similar. Generally speaking, Planets should have more diverse under-ground biomes. The more unique the planet is, the more unique the core should be.
I'd be all for this, but in addition I'd also like to see hollow core planets where the centre is just a big old cave with crystals lining the ceiling. You could get out your Grappling Hook to swing along the ceiling and mine yourself them thar shiny thangs.
