Looks great!
One thing I'd change is how borders look so very square. Would it be possible to make them look more curved? Or use a cross shape for influence like Civ4?
I'm not fearing city spam so much now as I am fearing city GLUT (spreading out so far all the cities are touching one another or nearly.) What's the use of pioneers when all you need to do is expand your borders?
I can see this happening too.
Suggestion: make it so the cost of founding a city is increased the further away it is from the rest of your cities and especially from your capital. Some formula like this, with more balanced numbers:
cost of new city = [number of existing cities - 1]*50 + [distance to capital*10] + [distance to closest city*10]
and then use the outpost idea and
cost of new outpost = [number of existing cities+outposts - 1]*20 + [distance to closest city or outpost*10]
Different factions can have different modifiers of this, too, to add some differentiation in city-spam levels. Like the expansive and organized traits in Civ4.
SMAC comes to mind where factions like the Hive [despotic, ascetic communialists] were brilliant for ICS [infinite city spam] while others were accustomed to building very few, but high quality cities, like the Morganites [corporate, luxurious free marketeers].