What wouldn't he do?
Perhaps it was less of a question of what he wouldn't do, and more a question of what he would do. How far would he go- how far was he willing to go- to protect his world.
He put up with the heroes' incessant annoyances (That's you, Tony) because it was what he would do.
He jumped at Nick Fury's every beckon and call because it was what he would do.
He worked in normalcy with Captain America- the hero he had practically idolized from childhood instead of just goggling at the man, which would likely have happened in any other situation. That was hard. He did it.
He put Jane Foster into protective custody so Thor wouldn't worry.
He started up the safety protocols when the helicarrier's engines went out for the count.
He retrieved the rifle (when he didn't even know what it did) so that he could face Loki himself.
He watched, felt, and heard himself dying at Loki's hands.
It didn't really matter to him. He planned it all along. To bring the Avengers together.
It was what he, Agent Coulson, would do.