(First of all, a tiny disclaimer. I tend not to use games with special combat systems. If the rules start mentioning handedness or grenade scatter diagrams, I'm outta there. So there may be systems out there that address some of what I'm curious about, that I'm just not familiar with).
Waiting your turn in combat scenes is often very dull. There are lots of solutions to this - Sorcerer's "each character must be able to inspire fanboy-levels of worship in the other players" for one - but let's ignore them.
In Magic: The Gathering, and cardgames like Gloom, players can play 'interrupts' during another player's turn. Sometimes these modify that player's action or cancel it entirely. Everyone stays alert and involved.
I'm thinking of a system where players could play 'Comeback or 'Dense' to interrupt the effects of 'Sarcasm', or 'Counterspell' to interrupt the incoming 'Fireball'. This touches on the trump mechanic in Dying Earth, where each social action is vulnerable to a certain defense.
So are there any RPG systems where players can do similar things? In Storyteller games, players can cancel and action and default to a lower initiative. But that's the only one that comes to mind.