It would be cool if they released content patches for Classic in the future. We have things like Uldum, Grim Batol, Hyjal, Gilneas, Emerald Dream and even Karazhan crypts that could easily double or even tripple the amount of content in Classic.
The only issue I see is that if they want to be able to progress into TBC at some point they can't go over the 115 iLvl of Karazhan.
So for raiding we could get something like:
* One 20-man raid between AQ and Naxx in gear
* One 40-man raid equal to Naxx in gear
* Two 40-man raids above Naxx in gear but equal to each other
* One 20-man raid between Naxx and the two new raids in gear
And of course lots of new dungeons who doesn't invalidate the previous ones but have some interesting gear here and there.