This is how I would state what a "future work" is:
"A work that has a reasonable expectation of being released at the time the question is asked that will likely to contain an answer to the question."
Now breaking that down we need to define a few more things:
- A "reasonable expectation" is: Part of a planned series that has been announced by the content creator/controller (e.g. Star Wars, Fantastic Beasts films, MCU films, A Song of Ice and Fire novels). The timing should not impact this, if a film is set to be released in 1 month, 1 year, or 10 ten years the question remains a "future work" unless something changes (e.g. the sequels get scrapped or a supplementary work like novelization/comic/TV show is released).
- At the time the question is asked is self explanatory, but to give an example, there were no absolute plans to create sequels to Avatar (the blue cat people film) so any questions asked (that did not have any other valid close reasons) would not be closed per the "future works policy". They could however be given a "we don't know" answer. In addition, questions that are asked and answered prior to future works being announced should remain open. A new answer can be provided if and when the canon changes.
- Likely to contain an answer is perhaps the most ambiguous and I fear the end result becomes a "I know it when I see it". However, it also the least important part of the definition. If we know a sequel is going to be released we probably know there will be more answers there. The exception is could think of is if the content creator states that a sequel will not a be related to the previous work (which would be odd but not impossible).
As I was writing this I saw a comment that is relevant and I agree with... If a question can be answered with existing canon, the future work policy would not apply.
Also keep in mind, that as we strive to be good stewards of our community, that we need to explain this to new users, and help them along in posting good questions. I would make it a point to any OP that closure is not permanent, and that if someone comes along with an answer the question can be reopened in short order. I don't have any data offhand to back it up, but my personal experience has been that we as community have a good track record of doing just that.