Remember the fear you felt the first time you heard a Fel Reaver in outland? Well I think that parts of the world should be periodically attacked by a main boss of this expansion. These bosses would be unnattackable and unkillable, and you'd have a little bit of warning to get the fuck out of the zone. Huge devastation would be unleashed by these bosses, I'm talking sure death but no durability damage and maybe a little rez timer that's 30 seconds so you don't have to run back. Anyway....
Kalimdor-Ragnaros: Since Ragnaros is up on Mount Hyjal in Kalimdor, he'd have the patrol over that continent. Your screen would shake and he'd storm through the zone screaming his head off, leaving a trail of fire and death in his wake. The fires would remain there for a couple minutes. Also, any players who have the unfortunate luck to be within 100 yards of Ragnaros are picked up by him and singed to a crisp, then tossed back down to the ground dead.
Eastern Kingdoms-Deathwing: I believe Blizz has stated Deathwing emerged somewhere in EK, so he'd be the patrolling boss for this continent. Every now and then your screen would shake, and all around the zone huge versions of those rubble things (like the burrowed jormungar rock things in hellfire) would pop up. Then you'd see Deathwing fly over and all around those rubble things there'd be a big earthquake and players on them would get thrown across the zone. Deathwing would also go to random spots within the zone and pwonatize them with fire.
All Coastal Zones (Eastern Kali+Western EK)-Lady Ashzara: In addition to Deathwing or Ragnaros, players in coastal zones also have the chance to witness an attack by Lady Ashzara. Once again, the screen would shake and she would appear, standing still in the water near the coast of the zone. She'd yell something then summon a huge tidal wave. It would pour towards the interior zone and knock players 80 yards inland when it hits them. Perhaps after the tidal wave some naga guard elites (attackable, with about as much health as a cata-level fel reaver) appear around the zone and patrol around ganking players for a few minutes.
These attacks would probably happen every few hours. Pity to the person who happens to suffer a simultaneous ragnaros-ashzara attack ;D