If the question is idle speculation about the continuation of a story, it indeed seems to fall under "primarily opinion-based". If it's not, then don't close it as such. This has worked reasonably well so far. In fact we had something similar to a specific close-reason before, but it was dropped in favour of more important and clear close-reasons, specifically because it was quite prone to misuse and not suffiently clearly defined while being easily covered by existing close-reasons. For more information, take a look at the discussion about its retirement. So at the moment I don't really see us sacrificing one of the other close-reasons in favour of a new one about "future works" (and asking for a 4th custom close-reason needs a really strong argument).
The point is, that you should always look at the specific question at hand rather than some whatever-natured definition of a "future work". The problem isn't with the topic or film that a question asks about rather than how it asks about that film. There are perfectly reasonable questions that can be asked about an unreleased film. And on the other hand you can very well ask a question that's idle speculation without necessarily mentioning any possible future work.
Getting hung up on if the question is possibly answered by some new film seems to be the wrong approach. If your question is asking for something that can't be reasonably deduced from the movie, it might just be "opinion-based". But it's a little odd to have your question closed simply because someone thinks it will be answered by some possible movie that might be released at some point in the future. Either the question is reasonably answerable by existing material or it isn't, but it doesn't seem to matter for the question if it possibly will be answered ever or not. We can see the problem this approach brings on this related discussion on another site.
Of course it isn't always easy to decide when a question could be answered by existing material, especially since an answer could very well be formed by informed speculation based on what we know about the story and the characters, but this problem IMHO isn't helped by the notion of a possible "future work". Yes, sometimes it is quite definite that it will be answered by a future work, like when you're asking about a trailer or in the case of questions asking about the ending of Avengers: Infinity War, but we still have to look at the actual question and what it asks for. If it's opinion-based at the moment, then close it as such.
I would also add, that we had quite a few such speculative closes recently pertaining to the new Avengers film (and similarly in the past with the last season of Game of Thrones). But honestly, this also has to do with the fact that the community seems to be a little fed up with the topic and with users looking for explanations to things that might be resolved later on. I can see how it would thus be easy for them to just slap a "future works" sticker onto each of those questions, but it's ultimately better to judge each question on its own merit and see if it is really primarily opinion-based, or just too broad, or entirely off-topic, or really not too speculative on a closer look (keep in mind that an answer might very well use existing story and character knowledge for informed speculation).
So rather than worrying if the question is really about a future film or not, close them as "primarily opinion-based" if they are idle speculation about the continuation of the story, or any other close-reason if appropriate, including a possible custom one you type in if none of the others fit, and leave them open if they're okay.
And in fact the specific question you provide as an example has its own kind of problems independent of the existince of a "future work". I don't see this so much speculating about how the story is going to continue. It is pretty much about the current film alone but the question's premise itself is already problematic in that it assumes that the characters themselves are entirely aware of their situation and what they would need to win the fight, which simply isn't the case. So that question is more similar to "why didn't the characters do as I would based on the knowledge we all have?" which is already quite opinionated in its very own right, regardless of its possible coverage in Avengers 4.