I almost did a 25-284 for my current rifle. But I wanted to be difficult and experiment with a true wildcat design, so I developed my QPC cartridge. Was a great learning experience. Ultimately I will probably make a short action cartridge based on the 284 case, but I’ve been following the project that Fuj is working on to see how it goes.I'm a pretty big fan of just about everything 284 so my first thought is 25-284 but being this will be a PRS gun and if you want something with a bit more barrel life I'd probably go 25 Creedmoor.
Either, would be easy peasy and both have top shelf brass available for them.