The current planet types as listed so far are: Earthlike Water World Gas Giant Lava Planet Metal World My thought is that there is more that can be done with this with comparatively little effort (the word comparative is important for later). While its great that all the planets are really different and represent unique aspects of game play, you could expand the game aestheticaly with the creation of like planets. With a reskining an earth like planet (or tropical or whatever) can easily become a mars like planet, a desert planet, an arctic planet etc. I heard talk of venus like planets with its rivers of acid they could be a reskining of lava planets. Obviously new art is a lot of work but they dont require the creation and testing of entierly new planet classes. However because the unique planets represent their own challenges thus far there is no reason why an earth like planet pallet swapped into a desert planet might have changes as simple as reduced frequency of metal deposits to constant attrition to land units or reduced land speed (these are palance issues and do require testing but less so than a whole new planet class, also constant attrition is probably not fun at all but it is an example).
I think the procedural generation and post-creation tweaking process used to make maps will already accomplish most of the aesthetic 'expansion' that you mention. This is only my speculation, however. Desert planets are already possible just by tweaking an Earth-type planet to have only sandy terrain. If you want desert plants you might have to add them in. Mars might require some color-changing. I'm not sure on the status of ice, but if you don't need snow mechanics, you can recolor sand or plain rock for ice planets. I imagine the game will already come with a great visual variety in its planets.
By the sounds of it planet archetypes can be modded. I suppose that wouldn't be too hard, either, so there'll like be a lot of planet type mods. ^ From the Kickstarter page.
