If you just have to have a bigger rifle, I'd look for a bullet with a similar BC as the one you prefer in the 6.5, then look for a case that can shoot it to similar speeds. That way the trajectory will match up quite well. Just some looney thoughts.
Now you've given me math homework...
You can copy off mine... My two main hunting rifles are a 6.5 CM and a .30-06. With my chosen hunting loads, the .30-06 launches 180gr Accubonds at about the same speed the 6.5 CM launches 140gr Accubonds, making for a pretty much identical trajectory. Of course you can use slicker bullets in the 6.5, but I've had nothing but good terminal performance from the Accubonds.
So I'd probably recommend a .30-06 for your second rifle, it shares a similar trajectory with the 6.5 when loaded right, and is the first larger chambering that really represents somewhat of a step up from the 6.5 (although there's still plenty of crossover).
