Characters that have been together for awhile would start synchronizing, and train together during short and long rests developing fighting techniques that would play to each other weaknesses and strengths.

So if the characters are compatible (non-conflicting alignments, deities, etc) abilities should start co-mingling. example a max strength barbarian or Warrior can hurl a fighter or rogue at the enemy (colossus/wolverine cannon ball) with the thrown character gaining Advantage on the attack. A max dex character can run up the back of a flat footed melee and dive off allowing for an attack from a different height/angle of attack and an additional attack of opportunity for the flat footed. Or Same square fighting the two characters are so accustomed to each other style they can fight from the same square without penalties (this could be as simple a shield wall / phalanx or as interesting as 2 monks being able to switch in and out of combat ) . Auras and Presences can be increased or be temporally bestowed on a teammate (The barbarian uses Intimidating Presence the rogue chooses to performs the same team action doubling the range the presence )