I agree about Rust and pretty much everything both of you had to say.
You're welcome for the original thought.
I could see it going one other direction though. Cliff Claven from Cheers bursts in at the end and violently orders a beer from from Marty, threatening him with postal fraud if he doesn't oblige. Being that they are fighting off a bunch of animal faced psychos at the time Marty slings a warm beer at Cliff Claven hoping he'll let him slide on it, Claven refuses it so suddenly Al Pacino burts in, tackles Rust and Cliff pees on Rust like in that scene in Two for The Money. Dude with scars launches a fireball killing Rust, Marty, Cliff and Pacino and starts celebrating with his friends in animal masks. He picks up the beer and gets pissed that it is now actually hot as opposed to warm from said fireball. Forgetting that the fireball was actually his own fault he has lightning bolts kill all of his friends in animal masks. He then remembers that he initiated said fireball, feels immediately ridden with guilt and has a giant foot come crashing in from the skies squashing him. His arm is all you can see sticking out from giant foot, and slowly it lets the beer go and beer slowly pours into the earth.
Fade to black, enter credits.