I'm with Mike! Just had to say that but seriously I'm not too interested in any of his suggestions except the .250-3000 Savage. Cool little round that is an awesome deer and pronghorn cartridge for youth IMO. The .243 has all but killed this cartridge off except for seasonal runs of ammunition.
I'd like to see the .338-06 brought back out in a commercial run. I wish Savage, Winchester, Remington, Ruger, and Weatherby/Howa would build rifles in it. Weatherby did build it but at $60+ for ammunition and $1200+ for the rifle priced it out of range of most shooters. If Federal, Hornady, Winchester, and Remington would produce ammunition in the $30 price range the round might have been more of a commercial success.
.358 Win is another I'd like to see brougt back. Ruger dropped it from the lineup a couple years ago, and very little commercial support in the ammunition department. Great little short range thumper for just about anything you want to point it at in the lower 48, plus it is efficient and works well in short (20") barreled rifles.