i must say that a real goal is actually what is missing to this game so far. Tho op is right, as soon as you get your citiy developped and most of your units maxed out ... you have not much left to do.
There have to be some more in depth mecanics with the pvp and globally what you call the "mmo-rts" part. Fighting for territories as a faction is a good start, also you can add some kind of pvp ladder to actually make it reawardind aside from getting crowns (which becomes pretty much useless anyway once you get all you wanted), the ladder would give a goal, and actually it could give ppl that meet certain conditions some privileges, such as ... building other cities or outposts (i read somewhere an idea about being able to siege those colonies regardless of army streght which i thought was pretty good) and the would allow to expend influence on other territories to make you faction "win".
The option to see other player's citys on the world map should be added too, (and camps) ... and i think the best way to deal with the fact that if there are too few players with too different army scales they can't fight ... just make a system life for the solo mission that allows one to select what he is bringing into battle so he can match his forces to the equivalent in strenght of the city/camp he is attacking.
This would smooth the pvp mechanics a lot, because sometimes it can take ages to find a suitable pvp match, even when you roam the map with 4 or 5 different armies of different strenght.