I’d like to see free exchange of weapons for those dropped on the battlefield (probably not any tainted by chaos though like Salp said, unless it causes your HP to drain steadily over time or w/e).
I’d also like to see the weapons have real weight, both in sound and the way characters handle them, especially bolt pistols and anything like that - the rounds should have a really visceral impact, both as they tear into the enemy and when they explode. Heavy ordinance should affect your movement especially while firing (seeing as these are guardsmen not space marines).
It’d be neat to have abandoned heavy bolter and auto cannon emplacements here and there too though I don’t necessarily expect it.
Less likely: I’d love to see some weapon modding in the form of exchanging scopes, power packs and other weapon parts.
Getting even more unlikely: It’d be amazing to see some vehicles show up, like hiding from a chaos-infused leman russ patrolling the streets while you have to snipe out its crew / melta the tracks and/or engine.