I think there should be a quest where the lich king takes you into his fortress (such as, maybe the Icecrown citadel attunement quest) and removes all your gear, and you get to sit there as he tortures other people around you, until he finally gets to you, and you start getting c'thun like whispers from frostmourne, telling you it's hungry for your soul and such. not just text, like, whispers that you can hear.
and then just as he's about to kill you, you get saved by jaina proudmoore, who teleports you to safety and reluctantly gives you the quest to accompany Darion Mograine, Tiroin Fordring, and the rest of the Argent Crusade on a final Siege against Icecrown.
The beginning of the raid would basically be an entirely phased icecrown, that would be only accessable to a raid group. the raid would have to use vehicles to fight their way through the endless hordes of undead, with a "boss" located at the steps of the Citadel. After a fight with the boss, you get the key to the citadel. The lich king appears and beckons you to join him, but before leaving, grabs jaina and disappears into the citadel.
Now, this is where it gets...new...the raid has to split, most likely in two ways. One follows Tiron, the other follows Darion. Each group has to help each other through a series of events, until both converge on the first real raid boss. With the help of Tiron and Darion, you defeat the boss, and soon after, the Lich king appears to taunt you. After the brief intercession, darion runs off after the lich king to exact revenge, while tirion takes a moment to let the raid catch their breath (drink, distribute loot and such), and when prompted (like arthas in the CoT: Strat) the raid sets off to follow darion. Down the next corridor, You'll find darion banging against a door, and while tirion helps with the door, mobs will swarm the area. Players must protect the two npc's until a new boss shows up, which won't be terribly hard to fight, but the door will only open when the creature is dead and gone.
After that fight and a few others (don't want to list ALL the bosses we'd have to fight our way through) we finally come to the Lich Kings throne, where he is just about to torture Jaina. Darion steps forth and instead, takes the blade for her. Through this act of self-sacrifice, he start to absorb as much of the lich king's strength as he can, before being tossed aside, apparently dead. The conflict then takes a turn, as Tirion, enraged and bewildered by the sacrifice made by Darion (which would probably remind him of the way his own son died) attacks arthas head on. This raid steps in and the fight starts phase 1 of the Lich King encounter.
This phase is a simple one, with Add's spawning through the lich kings army of the dead ability, which have to be offtanked and killed so the lich king can't death pact them. after about 20% the lich king stuns the room, before moving back to fight alone with tirion. At this point the raid has to Dps the lich king down to 50% before a certain timer reaches zero, and the raid wipes.
At 50%, the lich king stops messing around at this point, and blasts tirion away, casting a dark spell and stealing the life of the raid over 5 seconds, and reducing the raids health to about 5% stunning them, while healing himself up to full. Finally, just before the final stroke can be taken, Darion, with the ashbringer in hand (the true ashbringer, mind you) takes a hard swing at the lich king, taking his heallth down to where it was previously, before the lich king can knock him away. this is the "burn phase" where the lich king is finally weak enough to the raid to take him down. he has a soft enrage every 30 seconds, where he'll empower Frostmourne and try to such the life out a random raid members, who must be healed through it (or the channel must be cancelled by melee, otherwise the effect kills the raid member and raises him as a raid level zombie under the lich kings control.
Finally at the last phase, at about 25%, The leaders of the Alliance and the Horde arrive to help, and buff the crap out of the raid. Just as they help, the lich king calls on all of his power, enraging. This is the final phase, where you either kill the lich king in time, or your buffs fade and he destroys the raid.
After a final victory over the lich king, jaina laments over his body as his life ends. he tells her, going between arthas and the lich king, how he wished things might have been different (arthas with a good story, the lich king with succeeding in ruling azeroth). After all is said and done, Darion laments as well, saying his purpose in the world has been done, and all that is left for him are the memories of death. Tirion responds by telling him how much like his father he truly is, and that he would be proud. Darion takes up Frostmourne, which has lost much of it's power due to the lich kings death, and says the Ebon Blade will vow to protect the sword from the wrong hands, so that this terrible series of events will never occur again.
At least, that's my version