League of Legends' current theme is all about the Ruined King, Viego, and the next champion is all about another part of his wife's soul, Isolde.
To understand the lore importance of the next League of Legends champion, you need to know the basics of Viego and why he's all pissy about Senna. He used to be a rich brat that became king long ago and he eventually became obsessed with Isolde, the poor soul who eventually ended up being his wife.
While this seamstress was apparently fine with being a manchild's wife, she eventually fell ill and he sent his army's finest all over the world to find a cure. Kalista, his most esteemed general found a blessed isle with mystical powers but the residents warned her about misusing the benefits. Trying to revive the now-deceased Isolde would be one such violation.
Viego didn't care about that, he got Hecarim to betray Kalista, tried to revive Isolde on the isle and it became a corrupted version of itself, now known as Shadow Isles, the cursed land where the dead souls roam. In the process of all of this mess, Isolde's soul shattered into more than one piece, one of them ending up in Senna, which is why the bratty king still pursues her.
Now comes the part of the trailer below, with which Riot teased the next champion - Gwen. As you can see in the animation, one of the soul shards seems to have ended up reaching the doll Isolde made and it looks like this will be Gwen herself.
With this, Riot Games pretty much subverted everyone's expectations since the players expected Isolde herself to make an appearance. Instead, it will be another part of her soul that the snivelling king will be chasing.