Explanation
The system admin should consider joining the Open Source Commons & Community Sprints group in the Power of Us Hub as a next step for their custom solution idea. The Open Source Commons & Community Sprints group is a group that connects users who are interested in contributing to or learning from open source projects that are supported by Salesforce.org. The Open Source Commons & Community Sprints group provides access to resources, events, discussions, and opportunities related to various open source projects, such as EDA, NPSP, CumulusCI, or Metecho. The system admin can join the group and share their idea with other users who may want to collaborate on it or provide feedback. Posting the idea in the Trailblazer Idea Exchange, downloading and installing the project from GitHub, or asking colleagues to sign a nondisclosure agreement are not next steps that the system admin should consider for their custom solution idea. References:
https://powerofus.force.com/s/group/0F980000000CjQOCA0/sfdo-open-source-contributors
https://www.salesforce.org/open-source-commons/