It would be very nice and interesting if we could not "walk across water" but had to stay on land to get to a destination  - this would actually add some strategy and tactics (S&T) to the game. In the same idea of strategy, attacking from one terrain type to another should make a difference! That would mean that placing one's city could help make a difference. These are small things that could make a big difference in gameplay, It shouldn't be difficult to have or allow placing cities on mountains ( mountain tops become prime real estate), hell beans, this could help you make money as well, as long as you don't screw over the general populace. Adding at least a small level of S&T would expand your game in more directions - and your game is already one the better ones.

Also, I would love to see that jumping to a new site with one's town had a minimum time to stay there before jumping again. Most games I've played (over 20) require at least a 24 hour layover, very few require less, 12+ is the normal. This makes a S&T decision to jump-attack-jump to go away - because there can be a big cost to this strategy ( again, you could make this particular strategy of jump-attack-jump have a monetary cost).