You say: "when features look like they'd be useful to integrate into languages". How do you know a feature would be "useful" without compelling evidence that this is really a problem for programmers? Now you're back to the original problem of figuring out what the most significant problems are for programmers. Wouldn't it be better to figure this out BEFORE PL people plunge into a particular topic?
Remember all the research done on typestates? The motivation section of those papers was usually a few paragraphs of total BS. AFAIK, there was no real data nor experiment that demonstrated this was a real problem for professional programmers.
FYI: I like static type systems and used Haskell et. al. But no one has demonstrated that it is better than even Visual Basic!
Not really. The research, again, isn't in programmer usability. It's in formal logic and analysis of the semantics of computer programs. Benefits for programmers are just a side-benefit. The goal is advancing humanity's understanding of computer science, not in helping programmers, although sometimes the two goals are somewhat linked and deeper understanding occasionally yields industry benefits. In my example, and in many instances of expressive type systems, research has yielded tangible benefits for industry.
Remember all the research done on typestates? The motivation section of those papers was usually a few paragraphs of total BS. AFAIK, there was no real data nor experiment that demonstrated this was a real problem for professional programmers.
FYI: I like static type systems and used Haskell et. al. But no one has demonstrated that it is better than even Visual Basic!