Option to increase the variances in combat, encouraging the use of Combat Maneuvers in combat.

Basic-most rule change.

Certain or all combat maneuvers, when preformed with a weapon, against another armed foe will have a change applied to the 'Attack of Opportunity' rules.

The change takes place in the following circumstances:
 2 Conditions based on the attacker:
  You attacking with a weapon
  You must be proficient with the weapon
  You have a BAB of +1 or higher
 x Conditions relating to the defender
  Exempted if defender has the Improved feat relating to the Combat Maneuver

If the attacker meets the first two conditions and the defender does not meet the Exemption condition, then the following house rule applies.

The Combat Maneuver is allowed to take place before resolution on the Attack of Opportunity generated by attempting the Combat Maneuver.  If the Combat Maneuver is successful, the resulting Attack of Opportunity is negated.

Some optional rules.
Allow an exemption, or partial exemption from this rule if your HD is four or greater than the attacker's highest BAB bonus.  I.e.
Option 1: If defender HD is 4 or higher more than the attackers BAB, they are exempted from this policy and would generate AoO as normal.
Option 2: If defender HD is 4 or higher more than the attackers BAB, they are allowed an AoO after the combat maneuver, even if the maneuver was successful.

A few more options... applications...
Make a prerequisite of a specific a Swift Action to prepare for the Combat Maneuver.  Anyone with a BAB of +1 or higher can take this swift action.

A particular Combat Maneuver can only be preformed once against a specific target, in one encounter utilizing these rules.  Other attempts, the combatant is assumed to have become more aware of your style and is better able to detect the attempt.

A particular combatant can preform an intense study of an individuals combat style to make them exempt from this action.  This study normally involves actually seeing the combatant fight, and preform this action, generally at least as many times as their current BAB rating.  This viewing can be in person, or via scrying.  It is possible that by interviewing people whom were subjected to a particular maneuver could count.