You are definitely right that Sejuani should get a skin in this (hence my upvote), but please keep in mind that these skins don't usually come solo.
Let's assume that there will be 3 skins like last year: 
It was the year of the rooster, Azir was representing this pretty well while fitting in the thematic. But the other 2 champions, Vi and Garen, don't show the concept of the rooster, Even if there were champions who could have represented it in a better way. Examples would be Quinn and Swain.
(And I tell you, I would have loved to see Quinn aside from Vi instead of Garen....)
So, just one champ represented the zodiac, the other 2 were fitting in the theme to be Azirs guardians.
Same can apply in the year of the pig. Sejuani, representing the zodiac, should get the skin. But there should be 2 more champions at her side, fitting to her theme. They probably should be melee, since she could be their leader and they reworked her to be a great melee champion warband leader.
And it's not like there are any other champion, who would naturally come to mind if you think about pigs, besides Sejuani.
Have to admit that Kayn wouldn't be my first choice as well, though....
Looking for melee champions, who would go well with a chinese theme, I would pick:
Camille, because I personally think that she would be great with an asian theme. Hextech enhancements are not ruling her out like Vi has shown. Maybe make her legs look like they are working with clockwork, which china was pretty famous for?
Shyvana, I could totally imagine her in a chinese dress and she could turn into a chinese dragon. I know, people will argue that she would fit better for the year of the dragon, but the guy representing that year is probably Aurelion Sol anyway (if you look at the zodiac shirt from Riot). I struggle to find someone else....
