I've really enjoyed Spore thus far mainly because of all the variety and entertainment the different creatures and content provides. There's something special about making your own creatures and content instead of selecting out of a pile of x number of options or going down a particular appearance path like all other games have normally done. However, I've been left feeling underwhelmed by the lack of depth so I have some suggestions.
I would love to see the tribal and civilization stages fleshed out more. One of the things I think could add to it would be the addition of strategic play by introducing different classes/types of your creature. Imagine instead of having all of the same creature running around, there could be a variant that you could design to be a big and burly grunt, or perhaps a wild animal-mounted light and fast archer/gunner. As you gain more experience/prestige these additional classes/types could be unlocked for optional creation. Then, as you approach the civilization stage, these ground forces could be added to with the current crop of vehicles. Instead of individual control, they could be groups of multiple creatures. Essentially, blend the content strengths of Spore with the depth of Rise of Nations and I think that would make the tribal and civilization stages even better than they are now.
I'd like to keep the focus of this thread specifically on these two stages. I feel that the creature creation aspect of Spore is it's strongest and most entertaining point. Yes, vehicle building is nice, but the creatures are what generates all the fun in my opinion. It's disappointing to see less and less emphasis put on the creatures in the civilization and space stages. I feel a disconnect and, although there's plenty to do, the entertainment value seems to diminish for me so I'd like to see the tribal and civilization stages fleshed out, made more important, and complement the entertainment value the tribal stage introduces. There's a lot of potential in Spore, but I feel like it misses the mark later on because vehicles, spaceships, and cities just aren't as entertaining as creatures battling each other, singing, dancing, wandering about, and so on.