Personally the 6.5-300 Norma came to mind first.
And that's as good or better place to start than most other suggestions.
I'm not a fan of the cruise missile format of rifle cartridges. The STW, RUM et al, where you get to stick to the .532 bolt face but the cartridge lengths start to exceed 4" or better. Then you are also confronted with the distinct possibility of using them as a single shot...
I should acknowledge that this took place quite a while ago...
A small hand full of us pooled our resources and talents to form a small company which specialized in wildcats and barrels made to work with them. We were blessed with nothing but the the very best of clients who were looking to the future while trying to push the extremes of velocity boundaries and bullet construction, all at the same time.
We started at the top of course, .50 BMG but the lack of decent and available powder curtailed that pretty quick. But we struck pay dirt with the .50-378 WBY followed by the 6.5 on a shortened .416 Rigby case. This is what turns out to be the 6.5 Norma Mag basically, not exactly the same but in the family. It was a hoot to shoot!
Nothing really took off and we all had family obligations so practicality took over and moved us all around into more pragmatic forms of gunsmithing and design.
It's easier now because we can jump on the internet, order some brass and make a set of dies while waiting for the reamer to show up. We have actions designed for the Rigby bolt face, brass built to take some decent pressures and barrels made to stand up to those hot streams of plasma racing down the bore.
The 6.5mm on the Norma Mag case suits everything in my opinion. Even when using the longer solids or the heavy for caliber bullets, it's easy to accommodate either and still stay within the 3.600" boxes. We even have the option of going longer when using some of the DBM systems.
The only thing it doesn't solve is that persistent itching to try something different!