What are the best multiplayer games to try if me and my friends are looking for a similar experience to a Bungie Halo campaign marathon on Heroic+ difficulty?
PC. We played through all of the 3+ player ones around a year or two ago in chronological order and it was stupid fun, what we liked about it and are hoping to find elsewhere is it being a shooter with an emphasis on movement and it's physics sandbox (AKA CHAOS) while still having at least 3 full campaigns to play through in a continuous way.
I'm wondering if there are any other games/franchises out there that meet these same criteria, or at least come close?
The Half-Life franchise, maybe. Half-Life 2 and up has a lot of physics stuff including the Gravity Gun, but considering HL1 is free to play via Sven Co-op it wouldn't hurt to start there. You'll need something like Synergy or Obsidian Conflict to play HL2 cooperatively.