I don't understand what the problem is?

No two characters or players ever experience the game in precisely the same, quests can be picked up and completed in more or less any order. The only limitation really is what level you are.

It's always been considered that the levelling process is the representation of time passing in wow and what other players have achieved and experienced is more or less irrelevant to your characters experience.

Things will seem chronologically correct pretty much to all current level 80's on release just as much as it will for any new alts made. It will just be different.

I doubt there will be any npc stating at level 60 that illidan is dealt, Outland is ignorable and we midst march straight on deathwing. There will be nothing to tell low levels alts they should do anything but deal with the problems in Outland. Which, as a level 60 are still problems, Illi, kael, bash and kil are not dealt with directly or indirectly, in theory untill level 71 when the dangers in northren will become apparent.

Btw northrend is unnafected by deathwing and so is no different to Outland if it's a problem. Other than apparent overwhelming failure to understand the medium to which wow operates by some though I really can't see a problem here