Got in late on this thread, but I'd second the motion for a #1 in the venerable .303 Brit. I reload for this cartridge anyway (a Lee-Enfield #4) so I have a vested interest in this chambering.

Another #1 I'd like to see them do a run of (or re-run) is the .404 Jeffrey. But probably with the .450/400 out, they wouldn't.

The new "Boddington series" is bringing back my favorite "classic" cartridge: the 300 H&H magnum.

I have one from the original series, a limited 1989 run, and it not only sports great wood on the stocks, but it shoots very tightly. It's fallen easily into the "never-part-with" category.