So i am assuming you are rebarreling. The 338 wm barrel would sell well in the EE's, somewhat offset the cost of the new barrel. I am thinking of something in the 2.5"magnum family and the same barrel contour to simplify gunsmithing. The most popular would be 7mm rem mag. Course you already hit on the 264wm, then there is the 7rem mag, 308 nm/30-338, 338wm, 358nm, 416 taylor, and 458 wm.
Sticking in family should mean no messing around with magazine feed, or feed rails. Basically if you go with the same barrel contour, it is chamber a barrel, spin it on, check headspace, and go shooting.