Dave, how would it complicate the negotiation process to simply say "we are considering the following cities for future IETFs: does anybody know of an issue that they want to raise with any of these?" We don't have to say when, or how definite. It's hard to see how this would cause problems--can you explain?
I'm not objecting to just making an exhaustive list; I just am not convinced that it would really make any difference, particularly since things change. E.g., I suspect that the IETF that was scheduled for Charlotte, NC, is no longer in the offing... :}