Combat gets that much more complicated if you allow ANY kind of attack be used as an attack of opportunity.
That's all well and good, but it's a house rule.
"An Attack of Opportunity is a single melee attack" - PHB Glossary.
"As a melee attack, you may make a disarm attempt."
"You can use a melee attack with a slashing weapon to strike a weapon or shield..."
"You can try to trip an opponent as a melee attack."
They're all legal AoOs, according to the rules. You can rule otherwise if you choose, but it
is a house rule.
Grapple, Bull Rush, and Overrun, for example, can't be used as AoOs, because they're not defined as melee attacks.
Also, keep in mind that an AoO never provokes an AoO.
Do you have a reference for that?
To the best of my knowledge, if you make a Sunder or Disarm attempt as an AoO, or if you AoO an armed opponent while you yourself are unarmed, they're entitled to an AoO.
-Hyp.