To be honest, I hope they don't just keep on churning up expansion packs to raise the level bar higher and higher. Would you really want to start a new character after the 4th expansion know that you've got 6 months work ahead of you to reach maximum level? I'm not sure I would.
Instead, what I would like would be for expansion packs covering different aspects of the game. The one that springs to mind (and I'm saying this as a non-twink player) is a Battlegrounds Expansion pack.
It could be something like this - have an island around the size of The Barrens and on this island is two cities - one alliance, one horde. In each city are Battlemasters for various different battlegrounds (say 5 to begin with). However, the island and the cities themselves are world pvp objectives as well. If you raid the city and kill one of the battlemasters, then all the battles that master is watching are won by the raiders (ie. horde raid the city and kill a battlemaster in which there are 7 battles in play at that moment - all 7 finish and the horde win all of them).
To make the battles more interesting, rather than just have 'get the flag' as a main goal, actually have some of them to collect resources. So, for example, have a battleground set within a mine and the goal is to collect as much ore as you can (so - you'll be needing miners in your team). You could do the same for collecting wood, herbs etc. in different battlegrounds.
Ok, so this wont be everyones favourite expansion - but you can bet that all the twinks that play will go out and buy it (esp. if it doesn't follow on from TBC - so that anyone at any level can play) and the good thing for Blizzard is that, once they've sorted out the world pvp stuff in WotLK (with siege weapons and destructable buildings) there shouldn't be anything extra for them to develop - just to come up with geography of the island and the battlegrounds within it. No need to worry about developing quests, and making sure a quest npc is doing what it should etc. cos there will be no questing on the island.
They're talking about some sort of 'world buff' for the new WotLK depending on whos winning that pvp area, so they could also do that for here as well - although, and this is where it probably does get complicated, I would do it so that the faction with the most resources collected gets a price drop in the amount it costs for repairs, to be food, get stuff from vendors (because more resources means cheaper prices).