I think/hope it will have an optional DIGITIZER. I think it's important to make that distinction. A stylus seems to be mainly used for basic interfacing, which I think everyone agrees is better done with touch. A digitizer is mainly used for writing and drawing, which I think everyone agrees cannot be done (well) with touch.
I agree it's unlikely the digitizer will be backwards compatible with existing iPads. Try as they might, current third party styluses don't work well for one reason: iPads weren't designed for pen input. To make a reliable pen experience, the technology has to be designed and built into the iPad hardware as well as the digitizer, in order for the two to work properly in conjunction.
As for full Mac OS on the iPad, there are some major hurdles.
Mac OS X (and its applications) are not meant for touch input. Even if Apple redesigns OS X for touch, that still leaves all the countless OS X applications that have to be redesigned for touch by each of their developers. That's not happening any time soon. So it would mean the iPad would need a keyboard and trackpad attached pretty much 100% of the time, which just makes it exactly like a MacBook, and Apple already sells those.
If it was to be some sort of hybrid, running OS X and iOS, or OS X with just iOS apps, then the device would require two chips, since OS X (and its applications) run on x86 and iOS (and its apps) run on ARM. It would turn the iPad into a Frankenstein sort of a device, which I don't think Apple is too keen on making.
Not to mention running OS X requires a bigger battery than Apple probably wants to use in an iPad.
I think our best hope is that Apple beefs up iOS to have more OS X type features.
For me personally, if they just add split screen and multiple screen multitasking (with ample RAM), a proper file system (with apps that utilize it), and preferably a smart automatic method of keeping files in sync between the iPad and my Mac (without the use of cloud), plus the digitizer, then I'll be a very happy camper. But realistically for now, I'm just hoping for the pen and multitasking.
edit-
I'm also really hoping for a USB-C port, not only for speedy file transfers, but also because it opens up the possibility of using the iPad as a secondary monitor with my Mac, perhaps even with pen input!