Well, have you played GalCiv2? I mean, if you compare that and Civ4, calling "Or like Civ4 will we be able to create almost entirely new games?" - my point is that Civilization 4 is probably as far from intuative as you can get, when it comes to modding. I tried my hand at some light modding, and it just failed miserably. While in GalCiv2, not only is a lot of the nation/civilization creation functionality built-in, but there's actual, functional, mod tools (although I feel that the map editor leaves something to be desired).
I'm not a developer, and I'm short on official word at the moment, but the general idea seems to be that while the game may not be geared for a total conversion to be easy, there's going to be the potential for serious levels of customization, wheter it's unique units, civilizations, technologies or spells(?).
They're going to have a dedicated metaverse where people can easily upload and download mods of varius kinds, similar to Spore (although they had the idea before the release of Spore).
So when it comes to moddability, I wouldn't worry.
Although, of course, if there's no stock ridable non-magical bears as common as horses, it's all for naught!
