I like them going into the future to steal the song they've already written by that point, and bringing it back to solve the problem of not having written the song. It creates a paradox that pops up in science fiction a lot. Where did it "originally" come from?
It harkens back to them wanting to steal dad's keys, but they couldn't because he'd lost them. So they went back in time to before he'd lost them, and took them then. And it turns out that he'd "lost" the keys precisely because Bill & Ted had taken them into the future, where they were eventually found. I love the "time game".
Also, that teaser answers the question of what in the heck the "buff" Bill & Ted pictures were all about. Well, answers the "what" even if it doesn't quite answer the "why". But that's why it's just a teaser.