My favorite era of Warcraft is definitely late Burning Crusade.

Probably with Hyjal and BT as the top raids, Sunwell not released yet.
And needing to kill both Vash'j and Kael, before gaining access to Tier 6.
Our guild had Vash'j on farm, and could consistently get Kael'Thas to the four advisor stage, but not kill him (only two guilds were in T6 at that point).

I really liked the balance of heroics at that point.
Definitely a challenge for the later ones, not a walk in the park... but doable with a decent group.
And going by videos on this site, boxable too.
I was in the middle of my RAF, and there were probably a couple of months before WotLK launched.



I don't see Blizzard ever doing this.
But it would be fun.