General feature for insane pretenders would be the way to go. Rather than doing odd things or refusing commands, a pretender with insanity would intermittently make parts of the UI disappear and/or individual commands effectively cease to exist for a turn. This turn, you don't get to patrol, ah ha ha! What's that, you say? Search province? I'm sorry, I'm quite sure you mean pillage.
If you want real fun, it could cause numbers to misread or input incorrectly, too. Try giving a gem to a caster and they get three, but shows two or four, your research output is off by a few %, stuff like that.
Extra fun fun, you could start infecting weaker (lower domspread/strength) pretenders occasionally. "The dreams of the deep cloud your divine vision! Your formations are randomized this turn, and will be something other than what you choose should you try to change them." Or stronger ones if they (or their prophet) are actually inside a mad god's domain.
... just. Don't get it wrong. That is grotesquely horrible game design, and while as a challenge mode or something it might be interesting for the masochistic out there, anyone who actually tried to make that sort of mechanic part of a base game probably needs to be drug into an alleyway and beaten until either the stupid or their brain falls out. It's a heck of a thought, though.