This would be a very big deal in my industry - civil/environmental engineering. In the river/stream flood modeling space, the US Army Corps of Engineers' HEC-RAS program [1] is king. It's a critical part of FEMA's National Flood Insurance Program. HEC-RAS is free, but it's not open source, and USACE doesn't appear to have any plans to make it so.

HEC-RAS is a Windows-only GUI application. Supposedly USACE has an internal Linux version, not publicly available. HEC-RAS has a limited COM API, but it's not officially documented. I suspect that the API was exposed unintentionally. Most of the input files are text, but the format is very strange (very old-school), again with no official documentation. I spend much of my days reverse-engineering HEC-RAS file formats in order to make the process of building flood models more efficient and less error-prone. Other developers like me exist at competing civil engineering firms, working on similar reverse engineering projects and secret sauce tools for HEC-RAS.

If HEC-RAS was made open source, it would be a game changer. We'd be able to accomplish so much more. If the input/output files were officially documented, it would be a game changer. FEMA would benefit tremendously.

[1] https://www.hec.usace.army.mil/software/hec-ras/