The problem:

When you was about to attack/pillage an AI-civ whom you have established an peace-agreement with, a popup-message asked you if you really wanted to break that treaty. All you had to do was to click the Yes-button. Even trusted AI-allies could be shamefully double-crossed this way. The only thing you suffered was declining diplomatic reputation; some easy containable AI border-quarrels, and perhaps a lower civ-score in the Hall of Fame. Thats a waaay too weak deterrant-factor for most pro-civers.
Also, making diplomatic deals with erratic AI-civs where often somewhat of a hit-and-miss affair. Infact, both in Civ-2 & SMAC you where much better off never even try to "buy peace" at all, because any deals where not binding anyway. Everyone just doube-crossed everyone else - and because of this, diplomacy often felt rather meaningless.

I would like to have above replaced with time-limited, but mutually & exclusivly nullified unit-attack/pillage ability to all units that gets effected by that peace-treaty (they can still expel though). In other words: if you try to attack an AI-civ/unit from whom you have established a still valid peace-treaty with; a popup-message informs you how many turns is left of the peace-treaty, and one button to click: "Expel unit?".