It sounds like you've already got a shooter in 7mm, no need for another (though it never really hurts
). If you like the 300 WSM keep it too. I like the 300 WSM and there is no reason that you cant strech that thing a bit too, even though its a little lightweight. If you are think about getting a new caliber thats the way I would go just because diffrent cartriges get me more excited than a diffrent gun. With the gap you have to fill I would go up to a .338 or .375 down to a 6.5 or 6mm. otherwise there is always a .300 Ultra. If you were to double up on one of the calibers you already have I would go to with the 300 WSM just because I like em'.
What is you goal for this gun? Is it a heavy wieght 14.5lbs+ for quad/trucking in and then LR ambush style hunting? or maybe a little more versitile? My personal rule is never longer than a 28" finished tube and no more than 12.5 lbs with bi-pod and scope. But thats my rule becasue it fits my needs (backpacking everywhere). I may be moving to Alberta and that is flat, so I would build a 14.5+ heavy hitter what I dont wanna pack all day, In fact I may make it heavier.
The heavier and bulkier the gun the longer the barrel, the bigger cartiges can be used comfortably. if you live somewhere that is flat I would go with something Big, just because I could utilize it. So I guess to sum up, I think you gotta let us in on a few more perameters.