A robust map creating and sharing toolkit could make this game really thrive. To be fair, this feature, especially the map editor aspect, is something that would really work well if the developers implemented it. The ability to easily load in custom levels that you have made would also be helpful. For instance, you could potentially create levels in other software and port them over.
If we are going through the pain of adding in features that let us shape and edit the map, then we might as well add in new structures as well. Maybe they are castles from other games or movies. Forts from real life. Maybe they are completely unique. Imagine playing in a giant kitchen themed level that makes you feel like you are in honey I shrunk the kids or Alice In Wonderland.
Structures could result in the creation of tower defense-esque structures, like ballistas, cannons, or manned catapults, especially if you are allowed to dynamically create and place these structures mid-battle.