https://randsinrepose.com/links/2015/01/14/ask-them-how-they-want-to-grow/
Have an individual work on whatever the hell they want for 90 days. No constraints. Just their project. Extra credit for pairing them with other likeminded engineers. This is hard to pull off because there is always something urgent for these folks to do, but would you rather have them semi-absent for a quarter or absent forever?