I think it would be really cool to be able to have a sort-of echo sound effect in empty buildings or on a street. Same for a muffled sound effect when talking with stuff like walls in between two players. Same could be said for the squad chat, you could make it feel like a radio, where there's a bit of a static sound when a player is talking + a start/end transmit beep. The immersion would be so much heavier with these features, you'd feel like you're actually in combat.
PS: You guys should add an option to turn off player tags, purple shield, and sword to boost immersion more.