Whatever is released, it's guaranteed to upset people that have already made up their mind on what Apple *should* release. Apple would be insane to release Snow Leopard on a tablet due to the usability issues, so anyone wanting a desktop-like experience will cry that Apple is epic fail.
However, here's what I think is more likely, and I'd line up on day one to get it:
- General-purpose *browsing* device. Web browser, ebook reader, magazine reader. I can imagine people getting their Vogue or FHM subscriptions through iTunes, and using their tablet to flick through the magazine. (Fully interactive, hyperlink enabled magazines, mind you, not just static PDF's).
- Very slim, shiny hardware. Thin as an iPod Touch. Standby time counted in days. i.e. low-power ARM based CPU
- App Store for application delivery. Why break something that ain't broken (for Apple and customers at least. Dev's will still hate on it).
- One full-screen app at a time UI. Not neccessary limited to running one app at a time, though. Think full-screen office apps, powerpoint viewer, video players. Maybe use existing iPhone apps as widgets.
- Front-facing video camera, for iChat / Skype videoconferencing.
- Games. The iPhone has been massive for handheld games, this could be even better.
The above would be perfect for 80% of home users. Apple can position the MacBook / MacPros for professionals that need all that complexity, and the tablet for everyone else. With a stand and wireless keyboard / mouse, it would also make a cheap home computer, but it would be ready to pick up and go with you at any time. No complicated file system, no hard to use UI, no settings to worry about. Just works.
It's later than you think, the joint Russian-American space mission has already begun.