Plausible S3 Billy plotline
At the end of his fight with Steve in the last episode, Billy was left behind in the Byer's house. Also left behind was the demodog specimen in the freezer.
Though Billy ended the fight with far less damage than Steve, a vain kid like that would be worried about any punch to his pretty face.
I think its likely he could have woken up, found everyone and his car gone, and went to the freezer to find a bag of peas or something for his face. That is where he would have found the demodog.
That could be setting him up as a main antagonist for season 3, and make him a possible willing host for the demogorgon. Maybe it can sense his need for revenge.