I have few suggestions. It's only my ideas and you don't have to agree with them.
1. I think it should be cool if you would be able to see all your cities you have created in the world map. Just simple cosmetic feature. For example, If you playing for orcs you would be able to see your human and elven city in world map. Right now you can click on invisible spot and there is white borders. Well, maybe in a future they could even extend this feature. Maybe your one city could have impact on another. I heard soon you will be able to view friends town, so I think it wouldn't be too hard to implement this feature, what about if, for example while you playing as orcs you would be able to view your other city and maybe visit them with your orc hero and say hi to human hero.
There could be more options and impacts between your own kingdoms.
2. Make buildings look change a bit while you increase they level. Do you remember The Battle for Middle-Earth games? For example at level 3 barracks in that the game gets tower which can shoot arrows at the enemy. In DoF, for example Orc grant hut in level 2 could get second floor, in level 3 hut becomes stone hut, in level 4 it gets tower etc... Just a nice cosmetic feature.
3. In DoF there is summer and winter seasons. What about if developers add spring and autumn seasons? Of course, I believe this would not happen any time soon, because it would require a lot of work for designers and animators. They could improve existing seasons too. Add fogs after rains, make storms etc...
That's all. At least for now.
And thank you for a good game.