Well, you can look at the ballistics tables your ownself and ponder and dream and observe pretty much the same as I did when I was 15 or so. There's all sorts of great calibers and great rifles out there bigger than a 30-06. Lots of them work really well in fact.
The thing is though, if all you have right now is a 30-06 and you want to expand your rifle selection to 2, you have to ask yourself what you want to accomplish. That 30-06 will pretty much do anything you need to get done in fine fashion. Realistically, even a 375 H&H will only do the same job as what you already have in your hands.
So why not do something totally different? You already have the practical side covered, so get something fun and not totally necessary. Yep, you could get yourself a .458 Win Mag. You would be way cool having something to shoot buffalo and elephants and rhinos and dinosaurs and the like. Or even more radical and this is where I'm serious, look at a 45-70 lever action. Totally retro in that it's slow and massive and not a bolt action. But surprisingly useful in an old-school way. But a very serious gun that will do the job in almost every application you'll ever face. The thing is though, when you take them out to the range it will be 2 totally different experiences. And that's what most of us with more than 1 rifle are really after. Whether we have 3 or 30 different rifles, we like something way different than the one next to it.