Someone who can do photoshop illustrations, here´s an idea:
First pannel: 3 waterbenders. Left is master Paku making a clean water move like in the opening sequence. In center its adult Katara making a tiny wheel of glowing spirit water in her hands. Right is Korra spashing like in the opening sequence.
Second pannel: 4 earthbenders. Left is Kyoshi doing the opening her sequence move, ripping a flat stone away. Center left is adult Toph looking to the viewer throwing a metal chain. Center right is Lin, lifting stones like she did when got her bending back. Right is Bolin throwing some pro bending disks to the right.
Third pannel. 5 firebenders. Old Iroh heating some tea, firelord Zuko throwing fire to the left, Mako posing with a flame in his hand, Young Iroh about to gettin into jet mode and Roku doing his xiaolin moves from the opening.
Fourth pannel: 6 airbenders. Monk Gyatso throwing some pies at the left, airbending robe swag from Tenzin, the three airbending kinds in air scooters and adult Aang throwing some pies to the right
At the top center is Long Feng with miniature or chibi dai li. Just down him is mad Azula and phoenix king Ozai surrounded by chibi imperial firebenders and blue flames. And bottom center is Tarlok, Noatak/Amon and Yukone, tith water waves and at the crest of them just a few lines of red. This order makes the effect of 1 earth villain, two fire villains and three water villains....