I think there are a couple of ways to respond to this suggestion.
First, the most practical and immediately relevant: this is something you would have to take up with the Stack Exchange developers, and would likely need to be a code change, and would likely impact the entire Stack Exchange network. All to deal with a single, subjectively-assigned category of question that you're asking be treated "differently" that all other questions. Regardless of what any of us in SF&F think about the idea, I'm betting your suggestion would get no traction at that level. If this is really a problem for us, we'll have to come up with a way to fix it that we have a real chance of implementing.
(@Keen's suggestion to just flag them for undelete when you see them is a good start, and I'm not sure we can do any better right now.)
Second, I'm not convinced that it's really a good idea. We are effectively setting up a category of questions that will never get deleted, many of which (IMO) are terrible. The bigger problem is, I can't even tell if it's a good question yet, so I don't know if I should upvote or downvote it. Suppose you saw the trailer, and thought "huh, I wonder who that character is?", so you came here and asked. And then you watched the movie and they show up in the first scene with a "Hi, my name is ___" name tag on. Your future-works question now becomes a bad question, deserving of lots of downvotes. But if you saw the movie, and you still don't know who that character is, then it's probably a really good question, and now it might even have an answer. But there's no way to know that ahead of time.
Lastly, just in general, as my own personal opinion (which I realize is not site policy, but there it is) is that future-works questions are silly. I understand that people are often antsy to ask questions as soon as they think of them, but asking a question that has a near certainty of not having a canonical answer is just a bad idea. Even if we get the answer from leaked scripts, or unofficial blog posts, or whatever, who's to say the answer doesn't change before the final product comes out? How do we even know we're not being intentionally lied to? Until it shows up on-screen, there cannot be a right answer. Is it really that hard to just be a little patient, watch the movie like everyone else, then figure out what you want to ask about it?