The main interesting issue is that 40S&W has turned up as an option. There have already been some rumors about 40S&W getting its foot in the door, as far as military use goes. This seems to be another indication that the round may be type standardized by the military for limited use soon.
As for the possible options, I personally don't see the USAF going with a current production Glock, due to the lack of manual safety (I think they could get by with the full sized and compact 40 cal Glocks as far as many of the other requirements go). But, I've heard the JCP submission from Glock was going to incorporate a manual safety on the Big Army version, so they may still have that one ready to go.
The G21 won't cut it for the USAF (further hurt by a lack of compact, rather than subcompact 45s), given the number of small framed women in the Security Forces and elsewhere who'll be end users, but they could probably be a contender with their .40 cal full-size and compact.
That said, the desire for adjustable grip sizes and the ability to set the pistol up with various action types makes it sound to me like someone still wants the HK 45 that was in the running for USASOC.