Yeah, I've thought about that as a possibility, too. It's definitely a reasonable idea. Along the same lines, another possibility I have considered would be for an influential stakeholder to publish a "Request for Proposal" (RFP) in order to attract someone who could fulfill a need to submit their own proposal.

The drawback to the bounty is that you have to trust someone to hold the funds (unless it's mixed with some sort of escrow/multisig arrangement). The drawback to the RFP is that the person publishing the RFP may not be able to guarantee that any proposal would be approved.