Power2people.org is not just for AROS. Try them out.
I'd write a proposal first, and have it discussed in some forum. That way, you get some feedback: there might be things you didn't think about, there might be something in the works already, demand might be smaller/bigger than you anticipated etc. You also get help in nailing down the bounty requirements - the more precise these are, the better the chances for a peaceful outcome and a decent release.
And whatever you do: the end result needs to be compilable by everybody with a simple make (or something like that) and *all* of the code needs to be released under a free license.
Personally, I'm not a big fan of bounties. We had a similar discussion on a German forum recently, here are some numbers I digged up for that cconversation:
Check amigabounty.net's
list of finished projects and see how many of these projects were actually useful in the end. Now add the numbers in the 'Bounty' column - the amount is close to 18.000 Euro. Plus 3100 Euro for Sputnik (see 'Closed Projects'), 4100 Euro for the source release of a 20 year old file manager (see power2people.org), 7600 Euro for Odyssey source release, donations for having the stuff ported to OS4 etc. etc.
That doesn't look like a good deal to me. And how many of these projects are still actively maintained? None, actually. I don't think the bounty system has done OS4 any good.