6.5 is about as multipurpose and flat shooting as it gets. it is a little light for larger game like elk but far from anemic. 303 brit and 54R are great at mimicking the ballistics of 308, at least to the lower end of the spectrum. 45/70 is a great casting/reloading round due to it's great versatility, allowing everything from bird shot to 420gr slugs. 44 mag is also quite versatile though it is only suited to medium game and smaller but a rifle/handgun combo in 44 mag would offer a great deal as far as personal protection and food collection purposes goes.
I'll agree that the 6.5 is very versatile, I wouldn't say it's a bad cartridge for large game, but youre limited to the heaviest bullets for the higher SD to break through those heavy bones and shot placement will be more important than ever.
Also a 45-70 can be loaded from birdshot up to 630 grain round nose slugs. Yes they go that heavy. You can have a subsonic round with over 1200ft/lbs of energy. Pretty impressive, or you could load it "hot" and push over elephants.