I have a two part suggestion for bounties. To be able to increase the amount of a bounty after issuance and to increase the duration (for when it gets close to expiring).
Example:
- Increase bounty size (an additional 50% cost) - allows you to raise the bounty for an additional 50% extra charge
- Extend bounty duration (increased 25% cost) - adds an additional 7 days (maybe 14)