I could see Walt doing a Madson one year type deal with Boras for Bourn. I don't think this is nuts if Bourn doesn't like his longer term offers.
Both players would cost some dough but the contract is a one year fix. No longer commitment.
If they are too rich, I could see Nyger Morgan. Had a bad 2012 but usually hits righties. Could platoon with Stubbs or Heisey. Would be a cheap free agent. Could see a trade for Eric Young Jr. Rockies might want young pitching, might be tough to find a match, but he improved last season.
I started a thread called Centerfield 2013, it lists a whole number of possibilities beyond these.
Key is a guy who plays the position and gets on base against righties. Powerhouse not required.
I don't see Fowler who would probably require a big return. Guessing they won't land him. The Chisox have another guy, D'aza, I think he may cost too many prospects. Bosox might want too much for Ellsbury, an injury risk.
Parra of the Dbacks is interesting, might cost Gregorius. Don't know if Reds would do that, but they might. With Hamilton coming, wouldn't necessarily give up Didi for CF, but Reds might if they like Parra. Dbacks might covet Didi.
DeJesus is another possibility, there really are a bunch of CFers who hit lefty on the market or possibly so. I wouldn't give up a ton with Billy on the way, but lots of possible guys.