Welcome....First off, I think you have made excellent choices thus far - you may not need anything. Your question seems to be more about chambering choice and general configuration & weight rather than any specific make and model, so I'll go with that. Next, keep in mind that I'm no expert. I'm a big hunter, but never hunted Alaska - you sound more experience than me in hunting. I won't mention anything like a .22lr or .223 or shotgun, nor a heavy rig like a "tactical / heavy precision" rifles, since you sound all about the big game hunting - which is pretty much where I am. To give you my best two cents, I'd like to know your age, fitness level, and what you see as the specific roles (species / continent / terrain / situation) of each of those 4 rifles you already have - also, relatedly, what actual accuracy are you achieving with each of them? .75, 1, 1.25, 1.5, 1.75, 2.0, or more (in MOAs)? I'd also have to know whether you want to go to Africa or not, and which country (if you know), or at least what species do you think you can afford to go after? Obviously, financial resources is a huge important variable, too. Also, do you reload or just shoot factory?
Without knowing more, here are a few possibilities:
1. Sell all but the .308, then get another one exactly like it as a backup. KISS principle. You could follow the exact same plan with that .270 - just make sure the backup likes the same loads as the primary.
2. Trade out the bush rifle for a similar config but different chambering: Either (a) a lighter-recoiling but equally effective rig in .338 federal, .338-06, .35 Whelen, or .350 Rem mag, or (b) just be done with it and go for a short-barreled (22" or less) .375 HH or 9.3x62mm, which can also be used in Africa to good effect on plains game and larger.
3. Get a true high-alpine mountain rig: trade out that heavy-ish .270 for what is really the ultimate in long range for sheep, goats, desert pronghorns & mulies - which is the Kimber Mountain Ascent in .280 AI - really there is nothing not to like about this rifle (except Kimber customer service I guess - does it still suck?).
4. Your 'true stopper' rifle: Get something in .416 Ruger - probably a Ruger of course, in the Alaskan style - could also be used on anything in the world, including African DG.
5. Drink the koolaid and get something in 6.5 creedmore, to either replace or supplement the .243 and .308... maybe a Bergara, Browning X-Bolt, Tikka T3, or Kimber 84, among many other good choices. If you can't stomach detachables, ditch the X-Bolt and Tikka.
6. Catch super-magnumitis like so many people and get a hunting rifle in .338 Lapua, .30-.378 Wby mag, .28 Nosler, .26 Nosler, .30 Nosler, .300 RUM, etc.
Note that I wouldn't recommend #6 but many do. The only thing I might think a good idea on #6 is a .28 Nosler - overbore but not THAT overbore, and has a lot going for it as a fantastic long range hunter if you can afford to replace the barrel at some point and shoot the ammo. .30 Nosler has some appeal too.
Scopes - your configs sound excellent - look at the Leupold VX5-HD if you go for a heavier bad-arse LR rig. VX6 as well. Schmidt & Bender, US Optics, and Nightforce are always great options, *if* you can find one light enough for what you're trying to do (how far do you hike from the pickup and what's the budget?).
If you're willing to go used, your options open up greater still, in both chamberings and rifle choices.