Who decides what kind of work gets a bounty? Do you have stake vote for someone proposing what we would like built? I guess you could do that. But its just a direct mirror image of the DHF. Instead of proposal makers being builders asking for funding for their idea, someone proposes an idea and someone else builds their idea.

I dont think anything worthwhile can be built with this system as its by default too competitive. No one will put any meaningful effort into it to fight with someone else for a bounty. And if it becomes the case you have only 1 person/ group doing work, you might as well have them run a normal proposal.
Could be useful for small tasks that take tops day or two.

The milestone trigger is something that would definitely be useful but it doesnt really have much to do with a bounty system. It can be implemented into the DHF. Theres definite problems with it like getting cheated out of pay so you need some kind of oracle thats not vote dependant. But its doable already..