So this is a what if where Nya was born a fire elemental and kai was born a water elemental

Start the what if:

So Nya now has Kai's personality and Kai now has Nya's

Story starts now:

Nya is trying to make a katana, but fails. Then Wu comes to the blacksmith, Nya and Wu argue for a bit, then Wu leaves, then SKELETONS ATTACK THE SHOP, the same scene, just swap Kai with Nya and swap out with Nya with Kai. Since Nya is the fire elemental, they kidnap Kai this time. So Nya is trained to be a Ninja, Everything try stays the same, but this when Nya has to "survive the night", Jay and Nya get early feelings for each, After that they go on the quest for the Golden Weapons. The way they get the Scythe of Quakes happens the same way, but since Nya is not Kai, Nya doesn't use the scythe, so they just run out the cave instead.

While on their quest for the Shurikens of Ice, Jay makes a joke which no-one finds funny except for Nya. The events for getting the Shurikens of Ice and Nunchucks of Lightning stay the same. But with Sword of Fire, Nya leaves the team to go after Kai to save him and battle Garmadon, She saves Kai but cannot defeat Garmadon. Then Wu shows up to save them, then Wu goes to the Underworld. Kai then sees a dragon, then events with the other ninja, Jay loses his voice due to Cole hitting in the neck.

Events from their on the stay the same just swap out Kai for Nya. 

Also since Nya meets Jay, they show early feelings, so if you got Jaya in this, comment.