So, what can Blizz do to keep us interested in 5 mans? New content is expensive and time-consuming to create, and they've got that entire expansion thing to worry about.
But hey, there's a ton of heroics no one runs anymore. The Burning Crusade heroics. So....
Update bosses and trash by adjusting their stats and abilities
Remove all the rep requirements. Let's face it, they're confusing and a pain. Blizz has moved away from pain in the neck stuff, so get rid of them.
Add them to the random heroic dungeon finder.
Now, there's one problem with this. The level 70 capped guilds get gear from these heroics. You can fix that one of two ways. Either groups with level 70 capped toons automatically end up in the "old" heroic, or you have the option to run the "old" heroic. And as far as removing leveling content, well, you're really not. How many of us go run BC heroics when we hit 70?
And you could do other things too. Since many of the Burning Crusade heroics were harder at level (so I hear, I've never run one with a group completely at level), why not make them loot pinatas depending on how hard they are? The easiest ones could drop normal heroic loot, ranging up to icc 5 man level for the hardest.
Now this would lead to a bit of a desertion of the Wraith heroics, but a) the random dungeon finder will keep sticking us into them, b) players will go for the new and shiny regardless, and c) most of us have running the Wraith ones down to a science and will likely still turn to them for quick badge runs.
For those of you who ran these dungeons to death in BC, well, not everyone did that. I can think of only one BC heroic I've run anywhere close to level, and that was Magister's. I still want that phoenix pet, gorram it. Even if you've run these things to death, you've probably run the Wraith ones to death too, and far more recently.
If nothing else, making these heroics viable again might help with the pre-expansion slump, and certainly makes Blizz look better. Right now we have Ruby Sanctum incoming (but no new 5mans that I'm aware of), and then nothing till Cat, whenever that is. Given when the ICC 5mans came out, that's a fairly large gap in content, preceded by an even larger gap in content. And you could even dole them out a couple at a time, maybe keeping which ones are coming out under your hat so it's a surprise?
Note: I have no idea how hard/easy updating these heroics might be. I've never run most of them (I was 40ish when Wraith hit).