Design wise, I can't see any big changes for next year's iPad - maybe they'll save off a few millimetres and go back to the iPad 2's thickness.
I can see Apple keeping the current design going forward and only making changes under the hood, similar to the iMac/MacBook range - it might explain why they went with 'the new iPad' instead of iPad 3. The iPad 2 was so named because it was a true successor to the first model; an all new design as well as massively improved specs. Now however, any future changes are likely to be evolutionary rather than revolutionary.
The only practical change would be to make future iterations lighter, but beyond that the current design is probably as good as it's going to get.