A great "starter set" of long guns would be: 22LR, 30-06 (essentially duplicated by the 7mmRemMag, and a 12 gauge.

In the realm of rifles:

Going lower: 22-250 or 243 (or 22LR if you don't have one)

Going higher: 338WinMag or 338RUM (or 12 gauge if you don't have one)
[If 338 is just toooo alien for you, then substitute a 300WinMag or better yet a 300 RUM]

In any case don't just turn around and essentially duplicate what you've already got (gonna git). Once you have either the 270 or the 7mmRemMag, then a 280 or a 7mm-08 or a 308 or a 30-06 is NOT truly enough different to be worthwhile. IMHO space out your cartridge chamberings so your rack of rifles has some diversity and width/breadth to it. This way your "toolbox" can handle an array of game and hunting situations.

But honestly, you should worry about getting and mastering the 7mmRemMag first.

EKM