What about allowing learners to contribute to some kind of idiom "bank"? As we encounter idioms and expressions we could submit our definitions to be approved and added to duolingo's vocabulary.
Defining words and idioms is not something learners should be allowed to do. They simply don't have the expertise and experience to grasp semantic subtleties. A dictionary has to be 100 per cent reliable. Learners are bad enough at translating as it is. I have absolutely no doubt that a crowdsourced dictionary would have a significantly negative effect on the quality of translation. Having said that, Duolingo's current dictionary is severely lacking. The only solution I can see is to license a more complete dictionary with context information.
Fully agree. And I would go even further: even native speakers shouldn't be allowed to edit the dictionary. Compiling a dictionary is a subtle art and should be done by experts and only by experts. Licensing a "real" dictionary could improve the quality of translations on duolingo considerably. I rarely use the built in dictionary.