Been thinking about this while I went through the cataclysm content. At this point we go through the build-up leading up to Illidan, the Lich King and Deathwing but are cut off just before the action starts and sent to the next zone to go through that storyline. This is especially relevant in my opinion when it comes to WotLK and Cata.
So what I thought is if Blizz were to create questlines in Shadowmoon Valley and Netherstorm for Kael and Illidan, Icecrown at least for Arthas and then Dragon Soul or whatever (wasn't around for Deathwing.) where you accompany people like Maiev, Tyrion and probably the dragons then (again, wasn't around) or just go by yourself and finish that boss off in a similar manner as how the Sha of Anger and other boss-type entities in MoP were defeated but more epic and difficult but still not so difficult that you'd skip it.
You'd be rewarded with lots and lots of XP in order to make it worth it not just lore wise.
Of course the instances would still be around for people who want to do them, this would just be a phasing thing but it would tie the story together really well and come the final expansion it would perhaps even be possible to run through the game in a similar manner as Diablo.
So what of it, got any thoughts?