Spying rate can be changed. So much so it's possible to make it tear down your walls through sheer volume of spies thrown at them lol. I'll take class and unit level into consideration, but God knows how I'd balance that.
I like this idea. Prisons are cheap, but lvl3 towers are actually pretty expensive, and not all locations can build one. It sounds like a good way to keep kidnappings in check without trivializing them.
The switch idea is something I want to try, but don't really know how. Something that can easily be turned on and off in the game setting or something...
Army size is reduced to zero for kidnapped commanders, even if you rescue them shortly after. Making it so army size is preserved is... technically possible, I guess, but currently a pretty low priority. I know that restoring army size would greatly reduce frustration, so I'll keep it in mind.
And yes, the fact that your could randomly lose an important unit could be the cause of great frustration. Excluding Ares from being kidnapped isn't difficult, but as for everyone else, well... that's why I'm asking for thoughts and suggestions.
Curent factors being considered to balance the success of kidnappings are: level and class of the kidnapper, level of the kidnapped and tower level. I'm still not sure I'll use all of those, and even less sure how I'll balance them.
If kidnapping succeeds then the last resort is paying the ransom, the size of which would probably be affected by level of the kidnapped and... either total days elapsed in game, or number of forts under player control. Still haven't decided which. The ransom would also serve as a way to use some of the absurd ammounts of money you accumulate in the late game. But, once again, I dread the task of balancing that.
There's my mod, which you can find a few pages back, and there's Setcheck64's
Darker than Black, a difficulty mod. Setcheck also has another mod, but it's function has basically been subsumed in my mod. Those are the ones I know of.