I'll buy it if Apple can get developers like Adobe on board. Give it enough power (quad-core A8X should be similar to MacBook Air in performance), enough RAM (if the Air is getting 2GB then this thing should get 4GB), a pressure and tilt sensitive display for using with a drawing stylus (yeah, graphic designers and illustrators still use stylus, hence Pro), multitasking for side-by-side apps, and some sort of manageable file system that ties in with iCloud Drive and it could be a great solution for creatives on the go. A USB port with an API for developers would also be fantastic, though unlikely, even if it supported only a few devices or required MFI certification. However I wish it was more like 11.9" instead of 12.9".
What if the iPad Pro and rumored new MacBook Air are actually the same device? When you dock the iPad Pro to the keyboard, the keyboard serves as an extra battery, upclocks the CPU on the iPad, and switches from an iOS looking interface to a Mac OS looking interface, complete with normal keyboard and mouse support. If you think I'm crazy, Apple has a patent for an iMac design that does something similar: When you grab the edges of the iMac, it senses the heartbeat in your hands and disengages the stand, allowing you to easily lower the iMac to a steep angle more suitable for touch. It also transitions the OS to Touch mode, with bigger UI elements for finger-based input. So it's not completely unheard of.