I agree with Bassim, FLL could definately support widescale trans-Atlantic services. Presently, FLL's Terminal Four is the designated interntational terminal and South African Airways, Finnair, Air Transat, Canada 3000, Royal, Air Canada, Bahamasair, and Air Jamaica serve it. American Eagle, and Gulfstream International both also serve The Bahamas. Because FLL is 30-40 mins away from nearly 2 million people (Miami-Dade, Broward, and Palm Beach Counties), FLL is certainly capable of being a widescale spill-over airport from MIA. In fact for domestic flights it is and has developed itself into a strong destination for low cost airlines: Metrojet, Delta Express, Southwest, JetBlue, Spirit, Airtran, ATA, and Midway. So I believe FLL could easily support daily service to London, Paris, Amsterdam, and Frankfurt. Really any airline that serves MIA should also consider FLL, and market to the entire Southeast Florida populace. So my vote goes to FLL.
As for BDL , I would like to see TWA start 2x daily service to their SJU focus city before any trans-Atlantic service is begun.
CMH is a tough call...would agree that its porbably 5-8 years ahead of its time.
SLC is a good idea though, at least nonstop to London and CDG (DL/AF codeshare).
STL as I've said before needs stronger trans-Atlantic service, to compliment daily LGW and CDG. Would like to see FRA started, NRT in a year and a half, perhaps AMS, and even TLV to capture the Israel market in the western portion of the country.
Can not see PVD with trans-Atlantic service for a long long time.
I have a feeling British Airways and Lufthansa won't be the only two European carriers to visit PHX . Same goes with Denver...