First of all sorry for my english it’s not my native language.
This idea is rather simple, making defense PVE battle on kingdom when you can only use the kingdom card to defend it. The purpose is too make use of all the cards and justify leveling them not only for kingdom power. After that you can make more difficult match has the kingdom grow and punitive expedition.
As you level the defence teams in each kingdom gets stronger. That is until you get to a certain level (50 is when they start getting traits) where the kingdom caps out.
I set my defence with troops from all the same kingdom and change every couple of weeks.
Currently have a Wild Plains team in anticipation of the next event. 
To create more variety it could just be set that a minimum of two (2) troops from your home Kingdom must be in the defence team.
This would have the benefit of more defence team and kingdom variety. Leading to more variety in traitstone drops in PvP.
The idea is to have some kingdoms daily attack by the IA and you need to use the specific card of this kinbgdom to defend to it. After that it can upgrade your income (less criminality) or give you some defense reward if you succeed. It’s PVE content.
Another idea maybe to have a kingdom map inside every kingdom with point of interest after finishing the quest (following the exploration fight)
I have to say i found this game pretty amazing compared to a lot of other free to play, this game use a lot of thematics i enjoy, fantasy, card, simple match3, upgrading system (card kingdom and heroes)