i would love to see random units, what i mean like for a random unit such as a diplomat there would be Marco Polo, who get additional movement points & perform more effective actions. Spies, i have 2 in mind, James Bond & Austin Powers(i couldn't think of anything esle), they would hard to caught in action

as for military leaders, there should 2 types, one that is a pure blessing & mixed blessing. pure blessing ones are those like Joan of Arc, who just fights for the nation. mixed blessing ones are like Napoleon, they fight for you & if they do a damn good job, they think they should be the leaders & try to take over

Random leaders should have risks like taking control of a political parties(details in a thread called Government)& try to seize your throne. of course this would happen to the AI & over time these constant seizing of AI governments would create different leader personalities & would have constantly changing tactics

could use feedbacks

Seizing the AI's powers would be good, but I would be annoyed if they attempted to
seize my power away from me and it would thus make the game extremely boring.

You switch to a republic style of government (hopefully through social engineering ala SMAC) and then get this Napoleon guy as a leader. He looks pretty good so you throw him into the front lines and he really turns the tide for you. In fact, you use him to spearhead a massive expansion. However, once the expansion stops, the victories go to his head and he (if you get unlucky) starts trying to take power. Your civ plunges into revolution and, depending on various factors (Napoleon's popularity, the citizen's average happiness rating, etc.), Napoleon could take over and force your social engineering table to "Empire".

The danger of creating a "monster" could happen whenever you lean too hard on a leader whose personal outlook does not line up with your social engineering settings. I.E. if you had a democracy, but you keep using a fundamentalist religious leader's powers over and over, he/she could build up a certain number of revolution points and wham - suddenly your democracy descends into revolution and emerges as a fundamentalist state with that person as leader - or the opposite could happen if you set up a fundamentalist state but then start using some democratic leaning leader's bonuses all the time.