This Rumble is between the MCU characters played by North American actors and those characters played by actors from the Rest of the World.
Rules:
1) The Infinity Stones are not with Thanos, but they are in play. Red Skull has the Tesseract, Vision has the Mind Gem, Malekith has the Reality Stone (Aether), Ronan has the Power Stone and The Ancient One has the Time Stone. The Soul Stone is on Vormir and is therefore unavailable.
2) If a character is played by multiple actors (e.g. Hulk, Red Skull), then multiple versions of the character exist, each with only the feats from their portrayal by that specific actor.
I’m not listing all the MCU characters onto each side because I’m way too lazy.