I just bought a FN Mauser, that has been sporterized. It is a basket case, I get to assemble it myself, which is no biggie, but I can't decide on caliber. It came with two barrels, one new, threaded and contoured and short chambered, and an identical barrel which is already fully chambered and fit to the action. Both are 30.06.
Problem: I already have a 30.06 and don't really feel the need for another. I could do an Ackley on it I suppose, or a 30.06JDJ, but I'll never hunt with it, I'm left-handed, and it's not. I'm actually toying with the idea of doing a varmint caliber on it. Problem: I don't hunt varmints. There are a lot of barrels in the world. What do you think? Should I just 30.06 it and be done with it?

It seems you've answered your own question. If you have an '06 already, what else do you feel and need for? Are you into Highpower target competition? If so you could make a butt-kicking Open Class weapon. Or do you train new and/or female shooters, where high recoil may be detrimental? If so, why not chamber it for a light kicker like the 6.5 Swede? You'd have an excellent "first rifle" for newbies but one still very capable in the game fields.

There are any number of things you can do with your Mauser.