I thought about this fight, as a mix out of Yogg Saron:
Sylvanas, Tirion, Darion Morgrain, Jaina Proudmore are like the keepers
Sylvanas (-> Aspect of the Windrunner, Movement speed increased by 5% and Haste rating by 5%)
Jaina(-> Aspect of Magic, increases Spelldamage and Healing by 5%)
Tirion (-> Aspect of Light, decreases Frostdamage by 10% and prevents dead Players to be raised as undead)
Darion (-> Aspect of Shadow, decreases Shadowdamage by 10% and increases physical dmg by 5%)
Hardmodes are without those Aspects.
Arthas has the ability to raise dead Players as Ghouls/Undead/Banshee's who serve the LichKing.
When the help of Tirion is activated he will instantly handle and kill raised dead, with 1 min CD.
The Fight begins when all Players enter the IceThrone Room and Arthas says something like:
"Well done mortals, your journey was long enough, now you will serve the one and only king!"
Phase 1: You just kill many adds, like @ Kel Thuzad fight in Naxramas, but Arthas will freeze (dispellable) players who stand nerby and cast Death n Decay under them.
Sometimes he fears players with his deathcoil and heal some of the Adds.
Phase 2: "Well done heroes of Azeroth, now you'll fight a real Enemy!"
Arthas throws Frostmourn into the raid which is stuck on the ground, freezes the ground and deals 100k Frostdmg to every Player who stands like 20y away to each other. The Dmg is splittet up to near standing Players.
Now the whole ground is frozen and deals a identicly dotdebuff to the raid like Hodir.
Frostmourne rises out of the ground and starts attacking the raid.
Frostmourne is tauntable and does all 8 secs an Cursedstrike, deals 20k physical dmg plus 10k addiotional frostdamage.
The Cursedstrikes leaves a Debuff on the Tank which reduces Armor by 5000 and is stackable, the debuff lasts 10 sec.
All 45 seconds, Frostmourne flys up in the Air, targeting a Random Player and falls down into the ground. All Players and the targetet have to run away. If Frostmourne hits the or a player he will instantly die and Fronstmourne spawns a cold of the North AoE Dmg field (dealing 9k Frostdmg/s) on the ground.
When Frostmourn is about at 5% phase 3 starts.
Phase 3: Froustmourne flys back to Arthas, Arthas rises from his Throne and starts attacking the raid.
All 30 Sec. he Casts Unholypowers, Arthas sucks all life out of the ground and deals everyone who stands near to him 10k Shadowdmg/sec and heal himself with damage dealt.
As long as Arthas stands in this Area his attacks hit for 30% harder.
Cursestrike from Phase 2 continiues in this Phase and all 45 secs he will throw Frostmourne up in the Air similar like in Phase 2, but he will throw AE DeathCoils to the Raid (Fearing Players for 3 secs).
Everytime a Player dies (hardmode or Tirion CD) he will rise him as an servant of the lich king, Tanks/Melees as Ghouls, Casters as Liches and Healers as Banshees.
Every 25% of his Life, he will gain 15% Attack Speed.
At 25% of his fight Arthas starts sometimes make a whirlwind like Leotheras fight, when this whirl ends he's got a Aggroreset.
Just the second you bring him down to 0 HP, the 4 Aspects join the fight (even in hardmode) to give him the killing blow.
Nobody can Attack Arthas, see this as a little in fight Sequence.
There are little conversations like:
Tirion: Now you will fall, young prince
Jaina: Arthas, what have you become
and so on, Chris Metzen will do this fine :-)
Then you see Tirion, attacking with the Ashbringer Arthas, Ashbringer meets Froustmourne and a big explosion of holy light on the screen.
Ashbringer and Frostmourne are shatterd, Arthas dead body and Tirions dead body lie on the floor.
But then, somethings comes out of Arthas Armor:
The Soul of Ner'Zul, Phase 4 starts.
Ner'Zul deals a big amount of shadowdamage to the whole raid, maybe the tank needs shadowresistance?
But hes got big resistances to all magic schools, except holydmg where he's vulnerable for the double amount of dmg.
This Phase is the Dpstest, lots of healing to the raid, because he deals, death n decay, AE Shadowbolts, Blizzards and grounded fire (Ner'Zul was a shaman :-) and you have to bring him down as fast as possible.
When you bring him down, you can loot a Legendery item (only hardmode +0), the grip of Frostmourne (the blade himself is shattered), which starts a questline something like "The corruption of this sword is finaly dead, but when you hold the grip you feel massive powers in your hand).
Well that are my thoughts about an epic fight, please dont judge my Englisch skills, because its not my mother tongue.
Upgrades to my thoughts are welcome!