Additional Special Maneuvers / Attacks in Combat

Bull Rush

This house rule covers how to do a Bull Rush special attack.
Bull Rush is a full round action.  The only other action that can be completed during a Bull Rush is a Bonus Action. A Bull Rush starts with at least 10' of movement from the Attacker before coming into the Defender's square, which means the Attacker must start the round at least 15' from the Defender. The Attacker must be no smaller than one size increment of the Defender. 
The Attacker rolls an Athletics (Strength) skill check. This sets the DC for the Defender(s). The Defender may choose between Athletics (Strength) or Acrobatics (Dexterity) to roll to beat the Attacker's DC. If multiple Defenders are Bull Rushed, each one gets a roll to beat the DC. Once one defender has succeeded, the Bull Rush maneuver ends at that space.
The Defender's roll is modified based on the difference in size between the Attack and Defender. If the Attacker and Defender are the same size, there is no modification. If the Attacker is larger, the Defender has a -5 on their roll for each size category difference (Large Attacker vs. Medium Defender = -5 on Defender's roll; Large Attacker vs. Small Defender = -10 on Defender's roll).  If the Defender is larger then they receive a bonus to their roll of +5 (Small Attacker vs Medium Defender = +5 on Defender's roll). As stated above, the Attacker can be no more than one size smaller than the Defender.
Bull Rush does provoke attacks of opportunity from non-Bull Rushed opponents as the Attacker moves out of the threatened range. It does not provoke attacks of opportunity from those that are being Bull Rushed.