You have a show about 4 elements (wind, water, fire, earth) and there are 4 main characters that represent these elements.
You need to make a set of skins for that show and you see that the pet and 3 of the main characters have been made the earth, wind and fire character.
What should the last character be?
Captain planet obviously duh
Soldier 76 captain planet skin 
Theres actually way more non-bender people in the show’s universe than actual benders. Dont you remember the whole Amon arc where he was stoking up the flames of conflict between benders and non-benders? It was in the Korra show, if you didn’t watch that. Therefore, the 5th skin should represent a non-bender character, as they already have 3 benders. And yes, if Kiriko isnt in yet, then that last character should definently be picked with her in mind because she has to be in the line up. A Kiriko skin is essential for the well being of OW2 and the universe itself.
And besides, Aang counts as a water bender anyways. He is the damn avatar!
It definitely would have made more sense to have a katara skin as she is one of the most important characters in the show. I’m guessing they picked suki (a very minor character) instead specifically because they could envision a kiriko skin for her.