Given that the current plan is to keep the F-15C's until 2040 or so; I think that a purchase of something along the lines of the current Saudi buy of F-15SA's wouldn't be a terrible idea. I doubt the C airframes can last that long in the numbers envisioned.
Block 60 F-16's would also be nice, but not nice enough for a small-ish buy... I'd rather upgrade existing F-16's with the money if you're going to throw it at that fleet.
F/A-18E/F's... only if considering a purchase for the ANG. They'd be useful for the air-policing mission, and there are a good number of USN/USMC peeps in the ANG that would be familiar with the platform. But not for use by the active duty USAF.
Ultimately, though, unless this is an industrial play - keep a second fighter line open - we're better off sucking it up and buying the F-35's. They'll be cheaper in the long run, though that long run is probably 20 years out.