How to gather requirements from the crowd with hackathons

[Context and motivation] Today's software systems become more and more complex, especially when we think about connected systems such as cyber-physical systems or digital ecosystems. Customers thereby demand flawless apps and have several needs in mind that such solutions should provide. If these are not fulfilled, they do not use the solution. In such connected environments, usually many different stakeholders exist that all have different requirements.
[Question/ problem] In complex cyber-physical systems, the manifold requirements and possible solutions can overstrain requirements engineers and developers. What are ways to consider needs and requirements from different stakeholders? How can such input be used for requirements engineering?
[Principal ideas/results] In order to gather ideas, issues and requirements from several different stakeholders, we propose to consider hackathons besides well-known and established requirements engineering methods. With these, one gets to know his stakeholders, get real needs from his stakeholders, and get early ideas and prototypical solutions.
[Contribution] We share our experiences with two hackathons we performed in a research project that aims at developing a climate neutral city district with supporting digital services. We discuss opportunities and challenges and how results might be used for requirements engineering.