wondering if the new Mini will contain a quad core ARM. I think that if there's any Mac to try the ARM OS X experiment the this it it.
As others have said, what you describe is called the "Apple TV" and it would probably run something more like iOS or TVOS than OS X.
If Apple want to try the ARM on OS X experiment, I'd suggest a convertible (along the MS SurfaceBook lines) with an ARM in the tablet portion and an Intel in the base. When its docked, it could run iOS and OS X apps side-by-side, when undocked its just iOS with anything running on Intel "hibernated" in the base. That's really just a reaction to the current popularity of convertibles coupled with the fact that Apple (unlike MS) has a huge catalog of tablet-specific ARM software.
If they wanted to go all-in on OSX for ARM they'd need to go for broke and switch everything but the 15" rMBP, the Mac Pro and the 5k iMac to ARM (& with the stated aim of switching those when workstation-class ARM chips were available). Otherwise you're asking developers to produce ARM versions of their software to support just one model... and while, yeah, for modern software written in XCode and using official frameworks that
should be "just" a case of re-compiling, you don't do that without extensive regression testing at non-trivial cost.
I'm not suggestinh any of those ideas as likelihoods - just possibilities. The only real point of an ARM Mac at the moment is that it would cut Apple free from Intel and let the roll their own CPU/GPU combinations. I'm guessing a "pro" ARM would have lots of cores and make heavy use of OpenCL to offload work onto the GPU.
What would be
nice for the future of the Mini is something like this, but in Mac form:
http://www.intel.com/content/www/us/en/nuc/change-the-game-with-nuc.html - but I'm not holding my breath. However, to be fair, I think that chipset has only recently become available (it might end up in MacBook Pros, too), so who knows...