I can't remember if there's been any model for how the terrain will work in this mod, but I found some mock-ups I did a few weeks ago. One thing you'll notice is that I prefer 1x1 tile planets and stars to anything else - though perhaps that's just a preference left over from the CivIII mod, it's quite possible that some awesome-looking (and functional) 2x2 planets (or bigger) can be made with CivIV's graphics.
This first shot (taken from the Civ4 Editor utility) basically encompasses many of my ideas for how the game could work.
Of course, the form that cities will take is perhaps the biggest decision, and it seems that there are two ways to go:
"Star Cities" has the "city" built on the star, allowing the civilization to claim the entire star-system. The city then reaches out and works the planets in that system (for
both models, I picture the coast and ocean tiles - unless they possess resources/asteroids - producing nothing. Food/Hammers/Trade will depend on the planets themselves and the improvements you build on them).
One cool thing about this model is that it can take advantage of all of the tile improvements in CivIV; one planet could have a "cottage" on it, another a "windmill", planets could be customised with mines, farms, "lumber camps", etc.
"Planet Cities" has each planet settled seperately. This allows for a little more flexibility, and the ability to exploit special resources on each (is there anyway to, for example, get a gold resource off of a one-tile island without a city on it? I don't think there is, which hurts the "star cities" model). One thing you should note is that I placed the planets in these examples so that they do not over-lap (so your "city" on Earth won't also work Mars, for example). You lose the ability to build terrain improvements, however (because everything will have a city on it). Also, if we are to avoid over-lapping, this model can fit fewer planets around a star (maximum of four it seems, where the "star cities" allows for six before planets are joined to each-other).
I really don't know which I prefer right now, because obviously both have their advantages and disadvantages. What do other people think?