I know what you mean... anything between .243 and 7-08 is a "want" not a "need"... but nothing wrong with "wanting" one...
Between 6mm and 7mm, I would be inclined to go with one of the 6.5mm's over a quarterbore... since you are based on the .308 case as is, maybe a .260... but for a change up, perhaps a 6.5X55???
Funny I'm saying this... as I abandoned the 6.5's personally, but still have a couple Roberts filling the gap between my .243/6mm and 7X57/7X64... but I just really like the Roberts cartridge.