Very good idea but the "M" will be even further in the future.I have a few observations I hope you don't mind my sharing.OK here goes,you have a nice long list,and some very interesting choices.The problem is not all are suited to either a semiauto platform or are not feasible in the M.
First anything based on the 308 case: .243,.260.7mm-08,.338 Federal,and the .358 Winchester,are all good obvious choices for different purposes and cover a good part of the ballistic spectrum.Not to mention they all use the same bolt face,and magazine,all you would need is a barrel for any of them.The 22-250 also shares the .308 common .473 case head diameter as well so would work without requiring a new bolt.
The rimmed cartridges mentioned(45-70,30-30,.44 Remington magnum,.500 Smith and Wesson)all share the same problems,That being reliable feeding in a semiauto.Also due to case length,if the .44 mag and .500 S&W were to be considered it would fit into the XCR-L.
This one is purely my opinion,but I believe the RSAUM cartridges are on the way out.The close competition with Winchesters WSM's and market flooding,there just ain't enough demand for it.I'm not so certain about the future of the WSM's either.The .270 is the only one of those covering a possible caliber/velocity niche' that would be worth it to me.
I know the .270 Weatherby magnum falls very close ballistically,but it is a proprietary cartridge that is prohibitively unaffordable for allot of folks,not to mention ammo availability.The other problem is the R&D associated with these,because of the unusual case shape,they are not very conducive for use in semiauto's either.It can be done but it is more work.They too would also require a different bolt and magazine,making kits prices more expensive.
The grand old Ought six is too long for The XCR-M,unless designed for it which would increase cost and weight and wouldn't be worth the difference.The 308/7.62 NATO is the ballistic equivalent of the 30-06.Sure you can handload the -06 hotter in a bolt gun but in a semiauto its just not as practical as it's shorter cousin.The 6.5x55 Swedish Mauser is a darling little cartridge,every one I have ever shot was accurate as hell,but the rim diameter on it is differant the the .308 family and it's ballistically no different than the .260 Remington.So basically you would have the expense of a new bolt,and possibly magazine for nothing.I doubt that one will happen either.
The .300 Savage is another great cartridge,but it is a ballistic duplicate of the .308,so nix on that too.Although it does shares the same rim diameter,so in theory all you would need is a barrel.
This I doubt will ever happen,but I am a qaurterbore fan and as such,I think the .250-3000 Savage would make a good candidate,if it were more available as a factory loading.It shares the same rim diameter,so a barrel is all you would need.This cartridge I think could again be popular if Hornady would research it a little and use some of that new proprietary powder they have.The .257 Roberts isn't a bad choice either but I believe the case would be to long (7mm Mauser is the parent case) to use the heavier bullets in a magazine as it would infringe on powder space too much.
I hope I didn't step on anyone's toes by stating what IMHO are the practicalities of these choices.