While I think the map should still have an effect, I would prefer more terraforming options. Although I do not know how much this would actually be used in practice, it would be cool to have at some stage more options after some tech. In Civ II "explosives" upgraded settlers to engineers who could transform terrain (although it took ages). Tundra would still be worse than plains if one could build stuff on non-river or lake tundra tiles. Similar with desert. Why not irrigate desert to get one food, it would still be worse than most other tiles, but could at least transmit irrigation. Right now in CivIV the tiles that profit from adavanced tech/civic options are often the ones that were the best to begin with, especially river tiles.