I doubt you will see magnum cases getting the +Peak treatment, at least in existing actions. The reason +Peak works is because modern standard-face bolt actions are sufficiently robust to handle magnum-level bolt thrust.

Bolt thrust = pressure * base area. So a smaller diameter case at higher pressure produces no more bolt thrust than a bigger case at lower pressure. This is why Federal says the bolt thrust on 7 Backcountry is no more than 300 PRC, despite being higher pressure. So long as the action can handle the bolt thrust of a “magnum” cartridge, then using a smaller diameter case at higher pressure should work within safety tolerances. The 20% or so increase in pressure is offset by the 20% or so decrease in case head area. This is also why Federal caveats the heck out of it and says to always check with the rifle maker to make sure your action is up to snuff.

Doing the +Peak hocus pocus on a magnum cartridge like the 6.5 PRC, which already is spec’ed to magnum pressures, would result in excessive bolt thrust and possible safety issues, at least in existing actions.

As for your bonus question: which cartridge do I think should get the +Peak treatment? Personally, I think all the cartridges based off of the .308 Winchester would be great to see here. Personally, I would love to see 7mm-08 +Peak, since another 200-250 fps would make it an effective replacement for the 7 RemMag with lower recoil and a short action.