I always felt that the player has not enough interactions with Under-Cities and Pirate Coves.
The new Empire event asking the player to help an Elector Count by sending him an army gave me an easy-to-implement idea to improve the impact of these hidden lairs on the campaign.
Note : This post assumes that the detection mechanic is expended to and balanced for Pirate Coves (reduction of the cost and of the cooldown when you establish a cove).
When an Under-City/Pirate Coves is detected, a 4 decisions event pops ups with the following choices:
1- Send the garrison
: The player has to fight the garrison of the under-city with its own garrison. If an army is based in the city, it joins the fight. The size of the garrison depends on the location of the Under-City/Pirate Cove (major or minor settlement). A defeat only means the UC/PC stays (and your garrison/army suffers losses).
2- Hire mercenaries
: For a certain amount of gold, the lair is automatically destroyed (this is equivalent to the already implemented way to get rid of an Under-City).
3- Do nothing/Deny the threat
: Nothing happens (or public order penalties/bonuses)
4- Accept a bribe
: The player earns a certain amount of gold. The corruption (Skaven/Vampiric) increases during X turns.
If the city is not destroyed, the event will pop up again in Y turns.
I think the last two options add flavor to the game (especially if you want to ally with Skavens - which is a bit useless even with Dark Elves for the moment - or if you want to roleplay the Empire).
A nice addition to this event would be 4 new maps
: Major Under-City/Pirate Cove and minor Under-City/Pirate Cove. These maps could look like siege maps without walls. So a lot of "buildings" forcing the player to split the front line and fight for the streets.
Optional : The "reversed" idea could also be implemented when you own an under-city/cove and an "accidental" discovery happens.
PS : This is the corrected and improved version of a post originally in "General Discussion". I decided to post it here thanks to the approval and the insights of some members of the forum (original link : https://forums.totalwar.com/discussion/252710/suggestion-new-detection-event-for-under-cities-and-pirate-coves#latest)
PPS : Another link to a comparable idea I had with, in addition, propositions on how to improve Aranessa Saltspite campaign (https://forums.totalwar.com/discussion/247803/pirate-coves-aranessa-update#latest)