Can there be a a notification for those people that participated on the question in the past (either who asked or answered it) once it is made a bounty?
This would be a good feature to apply.
I mean when a question is featured, it's supposed to be featured and stand out from the rest to attract more attention. So sending one notification to all answerers and even the OP (if the OP is not the user who placed the bounty) to allow them all to have a chance to improve their respective posts for whatever reason which is needed (new updates, newly seen obstacles, etc.).
Hopefully this feature doesn't require a lot out of SE especially with the more developed sites with questions that have tons of answers (for example, Stack Overflow or Programming Puzzles and Code Golf).