A question in the future research section - is it considered appropriate to write a paper on it?
Just because an author puts forth a question doesn't mean that she or he owns that area of science.  You are free to pursue any question that was posed in or inspired by an article.  Even if the authors are actively pursuing the question, your research on the question will likely take a different form.  It is unlikely that you will have exactly the same methodology, so the results will be different.  And, even if the studies are exactly the same, the contribution you are making is important -- i.e., replication is essential to the advancement of science.  

So, if you are inspired to pursue a given research question, full speed ahead!