I have a hard time thinking if I'm going to put $5K+ toward anything that it wouldn't either be a 300f2.8 or 500f4. If you're just trying to shave money off the 500mm it seems kind of futile given that everything cost within a few grand of each other, if you can't afford the longer one you probably can't afford any of them.
What I would really like is a 100-300f4. If I'm going with long glass the 400f5.6 prime is hard to beat (especially for the price), if I'm going to use anything shorter than that, it needs to be faster too.
I have to wonder why they can't make medium/long zoom lenses go from f2-2.8 or f2.8-4. Something like that would be the epitome of multi-role lenses.