If you're considering going 7mm, then 7mmSTW would seem the simplest way, to get there, at least to me. THere are dies available from most of the major makers, RCBS, Redding, Forster, and Hornaday. As for brass you can use 7mmSTW brass, or neck down 8mmREM, 300H&H, 375 H&H. There is reloading information already available, from Barnes, and Hornaday, and Lyman too I believe.
I think if I was going to do a wildcat I would go with 7mm-300WBY, just because I have loads of 300WBY brass already.