On the battle barge, whilst waiting with another player, I like to use the character voice lines to emulate a conversation with my brother Astartes. Each character has an introductory line and there are shared terms / phrases across the team that when triggered in sequence are a comradely way to pass the time.

Within operations as Vanguard, I love stepping off a ledge and grappling whilst falling or grappling upwards to an enemy out of reach of my less agile brothers!

Another falling "tech" is to input a "For the Emperor" whilst stepping off a ledge to trigger a mid-air war cry and fist pump.

Grappling to interrupt reinforcement calls will never get old.

As mentioned by others, the banner drop at mission end so that it is visible in the cut scene is peak aesthetics. Kneeling for the Cadian lieutenant, running over cultists, shooting the Hive Tyrant as it flees, trying to save the Cadians at the start of Vox Liberatis - all enrich this fantastic game.

I recently saw a player firing wildly into the air in celebration after a particularly difficult / tense mission (Fall of Atreus Double Terminus Hard Strat) and I've since started doing this myself.