I would really like to see a 10mm carbine in delayed blowback or piston system. I'd buy that for sure. 45acp, meh, not really enough case volume to maximize the potential.
I would too. The only problem with delayed blowback and piston systems are the expense. I love the roller delayed system on my HK 91 but I would have to shell out a lot of money for a similar PCC (Always wanted an MP5 or knock off... In 10mm WooHoo!). Shooting my M1 carbine back to back with my blowback PC charger is night and day different in shooting experience... not that the PC charger is in anyway harsh.
It seems like the extra pressure and volume of gas would make 10mm a better candidate for such a weapon than 9mm. 50 AE would make me even happier and I would stretch my budget further for one. Since I reload the expense of shooting less common cartridges isn't near that of buying ammo off the shelf. I.e. The expense difference 40 S&W vs 10mm is pretty low when reloading. Unfortunately to be successful a PCC carbine kind of needs to be affordable to shoot for the average guy that doesn't reload.
My one and only AR(ish) weapon is my Sig MCX Spear which I purchased an additional Sig Rattler upper in 300 AAC Blackout for. The ATF pistol brace B.S. gave me a free SBR registration. I spent a fortune (a fortune for me) on my Sig setup... but have way less invested in AR(ish) weapons than many.. who have an arsenal of lower end AR's.
A 10mm or 45 acp PCC would most likely have to be sub $1000 for me to bite.