This could be for a bayou-themed event or a superhero theme (Swamp Thing), but I had the idea for Malfurion to be in a swamp-monster type costume.
There are already bog beasts in WOW, so Malfurion’s costume could be based on that. He could be covered in swamp moss, his arm wings leaves, and his head could have driftwood antlers. His staff could be a driftwood stick, his root ability become marshland, and his summonable treants could be turned into bog beasts.
Furthermore, a bayou event could have costumes such as: Hillbilly Junkrat, crocodile Dehaka (Or Diablo), Frog Murky, swamp witch Li Ming (or Jaina), airboat Sgt. Hammer, etc.