There's nothing else to a tablet if you're one of the scores of computer makers that has already done exactly that (tearing off the lid, keyboard, and touchpad and put in a touchscreen).
For Apple, I don't know. They haven't invented it yet and I've learned that they can be more creative with years of product development time than I can be with a few minutes of writing comments. But they sure as hell won't release something that isn't significantly different in some way than what's been released before in the tablet space.
They are probably holding off more for software that makes a tablet make sense rather than just hardware. The hardware is a known quantity, but software for a tablet form factor has IMHO never really been worked out well.
Sure it might make a great piece of kit for an artist to take around, but how big of a market is that?
I'm thinking that a more innovative approach would be in order for it to really work.
Of course they could go the other way - just make a really massive ipod touch.
I'd bet they don't release a tablet. It makes no sense to me.