To be perfectly honest with you there isn' t much of a " gap" in your rifles.

Looking at what I would hunt with the rifles that you already own:
.223 === Everything from prarie dogs to coyotes
.243 === varmints, prong horn antelope, deer, sheep
300 WM === deer, elk, sheep, mountain goats, moose, black bear, Carabou

I think you have all of the bases covered except for Grizzly, and Polar bear.

If you really want a rifle that would fill the gap that you think you have then the 270 winchester would do very nicely.

If it were my money I would think about a 338 WM, a 338-06, 35 Whelen, or something like a .444 Marlin or 45-70 that would fill the nich of the dangerous game category.