Generally, I think the tileset should be divided into interior and exterior portions. Interior would be all the grand halls, receptions, luggage recievement (conveyor belts!!), maybe stuff like offices or security rooms. Exterior stuff would be all the hangars, landing pods, metal grids, all the many spaceships and command towers. Also, for the hangar I'm imagining some interior could also be done, what with a background with rocket spaceships all lined up, ready to be transported to a launch pad for blast off. Generally I'm thinking anything regarding the spaceships themselves should be HUGE - but then, I fear we might run out of space in the tileset... unless we're either tricky or decide to make it TSF, which I believe many would not like (although I'm personally anti-tileset-conservatism). Lastly, I was thinking of some cool rocky desert backdrop for layers 6-7 or maybe just 7 (kinda like in the beginning of Egg Rocket Zone in Sonic Advance 1).