2 calibers come to mind, and both loaded with the same powder and bullets, and they're only about 150fps apart. It just depends on if you want to go standard caliber, or magnum caliber... You can buy good factory loaded ammo (Nosler) for both.
Standard - .280 Ackley Improved
Magnum - 7mm Remington Magnum
My next build will be a .280 Ackley, not only because it's one of my absolute favorite calibers, but because it will also complete my triad of standard Ackley calibers, as I already have a .25-06 Ackley and a .30-06 Ackley.
I handload, and I shoot the Berger 168 VLD's backed by 7828 SSC powder, and Fed primers in Nosler brass for my 7mm RemMag. I will most likely be using the same components for my .280 AI once I start that project. I just finished building 2 rifles, so I have alot on my plate for now with load development and stuff. It might be a while before I jump into another build. Hopefully everything will pan-out and I can build it before deer season 2014 starts.