I don't know if it's ever been done, but perhaps holiday-themed tournaments. Some ideas could be:
Valentine's Day: Only use entropy, fire, and light elements (pink, red, and white colors)
Christmas: Only use fire, life, and light elements (red, green and white colors)
Halloween: Only use darkness, gravity, and death elements (black and orange colors, and death just because it's Halloween related)
Earth Day: Only life and earth elements (self explanatory) [and maybe water too]
There are a lot of blue elements (aether, air, and water), so you might be able to find something for them (or make just a "blue tournament").
Someone earlier in the topic posted the idea of having everyone use duo decks based off elements that are "opposites" (so earth-air decks, fire-water decks, life-death decks, etc.), which I think is a really good idea.