Here's a "free" startup idea if someone wants to run with it: an opt-in feature bounty service for open-source projects.

Essentially, project maintainers could join the service with their projects, and then users could offer financial bounty for specific features/bugs to be resolved. The core maintainers would have right to veto any bounty, and once the corresponding pull request is merged the payment would be released and distributed, following a formula, between the implementers (the majority), core maintainers and the service itself (a small, possibly fixed amount).

I can see something like this as a core feature of GitHub, but an independent service could include repos on other platforms.