I find it rather frustrating that I can never tell exactly where I can found my cities without sending a settler there to see if it says "another settlement is too close" or whatever.  So my proposed solution would be an overlay that shows where in explored territory is a valid location to plant a city.  Alternatively, this could be integrated with the terrain window, with a little marker that indicates"Can found city here" or something similar underneath the text that tells you the terrain bonuses.