.284 vs .308, .024 difference...

.277 vs. .284, only .007" difference in dia.

Get a .300 WSM,

You don't need a 7mm mag or 300 mag to kill a deer at 500yds, a 270 will get the job done, but if you're talking elk at long range, I think I'd prefer a 30cal.

Also if you start getting into dangerous game...

Also in a big heavy rifle the 300+ mags are manageable, but do you really want to carry a big heavy rifle you have to ask yourself?

If so, maybe consider the .300 RUM, Wby,
I love the 338's, so maybe consider a 338win.mag shooting 200 or 210gr, it's flat enough, plenty of power, or 340wby in something like the accumark is a heavy rifle.

Lots of options, but if you already have a deer rifle/caliber, I'd want to step up more than going from a 270 to a 7mm say, I'd want a 300 or 338cal

Checkout the .300 WSM and/or .338 Win.Mag.