maybe the thing i miss the most about old multiplayer FPS games is just hanging out in a private server with a bunch of strangers and the game objectives feeling more like suggestions towards finding your own fun rather than a narrow focus on the outcome of a match. i loved getting lost in custom maps made by well intentioned designers who thought a little too big, 30 minute long sniper battles that don't advance the game at all but create compelling temporary rivalries, and maps timing out because neither team won but everyone is cool with because they're already automatically downloading another new map they've never played before at 56 Kbps.
