For my open-source projects, I have to rely on the users to send back bug reports, tell me which features they need, etc. I’d love to encourage all users to make a good-faith effort to do this via the license. Has that sort of thing been done?
If there is such a license, it would not be an open-source license. Such a license would likely fail the Desert Island Test
Can a group of people exercise their rights under the license, even if they have no way of communicating with you.
Besides that, a copyright license is not the best way to motivate people to contribute bug reports/feature requests/patches to your project.
A better way is to have a good README file that clearly documents how people can submit their bug reports, feature requests and patches and next to that a policy that people are responded to so they won't feed ignored.
With respect to my colleague Bart, a licence that "encourages" users to contribute back would not fail the desert island test: that is engaged when the licence requires such contribution or communication. The example Debian quote in the linked page refers to an author who says "you must hereby send me your changes"; note "must". A licence that asks the user to "try hard ..." or to "please consider ..." doesn't, in my opinion, engage the test.
That said, I don't think you should do this, for two reasons. Firstly, you shouldn't make such a licence, because licence proliferation is bad, and proliferating licences in order to add clauses that neither prescribe nor proscribe actions is even worse. Secondly, even if you can find such a licence, I think it's pointless to use it. No free software project I've ever used that (a) has any significant number of users, and (b) has an even slightly-usable request tracker, lacks for feature requests. In nearly all cases, the opposite applies: the tracker is packed with feature requests that are either infeasibly difficult or stupidly pointless.
If you want to maximise interaction with your user community, you are better off making sure the barriers to usage and engagement are low. To ease usage, package your software for ready installation on major distros (PPAs for Debian and clones, YUM repos for RedHat and clones, provide an APK or better still get it on F-Droid for Android, etc.).
To ease engagement, provide web fora as well as mailing lists, as some prefer one, and some prefer the other. Better still, gateway them to each other, so all members see all contributions. If you require CAPTCHAs or similar to minimise spam, make sure they work and are accessible for all users. A clear code of conduct has increasingly gone from being a very good idea to being a thing whose absence raises red flags. Be responsive and fair in moderation, so conflict doesn't drive some users away.
Make your project easy to use and a nice place to participate, and I'd be very surprised if you don't get a community.