In one of the best ideas I've read for some time, Cameron from Random Battle proposes that Blizzard should regularly create new "heroic" versions of older dungeons. He points out that when Wrath of the Lich King comes out, there will be 22 dungeons nobody goes to any more, and that isn't even counting the larger raid dungeons. Chances are that anyone who started to play World of Warcraft after TBC came out never saw Stratholme, and somebody starting after Wrath of the Lich King will never see any of the Outlands dungeons.
Of course the more different dungeons you have, the harder it becomes to get a group together for one of them, especially given the lousy current LFG system where you can't sign up for more than 3 of them, and "any heroic" isn't an option. So my suggestion would be the following:
1) Cross-server dungeons, including a cross-server LFG system, which works a bit like the cross-server battleground queues.
2) Every dungeon should have a heroic version, even the old Azeroth ones. Even the Deadmines! In most cases that is just a matter of increasing the mob's stats, creating new loot tables, and adding a badge of justice to the bosses.
3) When the level cap goes up, all heroic versions automatically also go up to the new level cap. So heroic Ramparts after WotLK comes out should be level 80, not level 70 any more.
4) Even old raid dungeons should have heroic versions. Hey, I'd love to revisit heroic Molten Core from time to time! The heroic raid dungeons from previous expansions should give less loot than the new normal raid dungeons, but should give badges of justice too. That could actually work as an ideal training ground for newbie raiders!