Attackers already have a few different types of triggers:
1) Revenge – Basically, keeper assassination
2) Stealing gold – You see this with bandits
So you suggested adding:
3) Stealing equipment – perhaps another trigger for bandits
4) Strategic advantage – Some tribes may want to destroy rooms, damage bridges, attack eyeballs just to keep the upper hand
5) Prisoner rescue – When a tribe member is taken prisoner, you might expect the tribe to try and get him back
These all make sense. Perhaps also:
6) Glory and experience – Killing a particular minion or as much of my population as possible
7) Exploration – Some tribes could be motivated to explore the map and then take a disliking to me, after a large number of turns.
More possible triggers could add variety without making the attacks more frequent. For example, existing bandit attacks could be split into two types. Gold looting and equipment looting. They can still only happen as often as they currently do.
Back to the current design, dragons have burnt down my library before, but only by accident I think. Some tribes eventually kill enough population to satisfy them and later go home. These things don't really happen very often. I agree that some more types of triggers would be more fun.
