Yeah, it's typical company policy to send out a "we can't look at your idea" email in response to suggestions. Definitely doesn't mean they won't do a BTTF thing eventually, it just means that that idea has to come from somebody within LEGO (or possibly somebody from Universal).
Every licensed LEGO set that I can think of, though, has been tied directly to the marketing of a current movie or TV show, so BTTF is a pretty unlikely choice. The only exception I can think of off the top of my head is Batman, and even that started around the time when Batman Begins was still a big deal. BTTF is a classic, but it's a quiet classic, so unless Universal does a massive marketing blitz for the 30th anniversary (way bigger than what happened this year for the 25th), I don't see it happening.
"I think it's the idea that counts, not the actual method. As long as you set out to cause a horrible death through insects eating the person, you're good." - apenpaap