Is it OK to publish research that was in someone else's future work?
There is no &quot;calling dibs&quot; in academic science.
In some cases, it may be risky/unwise to start a research program that is very similar to what someone else is working on, especially if they've already published in the area and you have not, but that is simply because they likely have a head start and you are at high risk of being scooped if they publish before you.
Otherwise, if you've completed work relevant to someone's suggestion for future work, that's great. They may have chosen not to pursue it for numerous reasons, so it would be a big problem if simply mentioning it barred others from pursuing a particular avenue. It's probably appropriate to cite their suggestion if you are aware of it, even if it wasn't initially motivated by that source.