Oh god yes!
Blizzard listen to this one for he is wise.

That'd be amazing, but how would you get loot from final battle? Meeting up with NPCs at the end, who shove more powerful weapons in your hands and push you back in?

I really like this idea. A 5-man Arthas fight where you don't actually fight him, but flee from him makes sense lore-wise and gameplay wise. It'd be a bit cheap if you did a normal fight with Arthas, with his HP and strength severely reduced for the sake of game mechanics. And to be honest, I think it would be great to fight a boss that's unkillable and you have to escape from them instead. It'd be a nice change from the usual tank-and-spank.

And Alex, you'd probably get loot the same way all non-killable bosses give you loot: a conveniently placed treasure chest at the end of the gauntlet.

I had a similar though about a chase scene, but kind of the opposite of what you said. Instead I kind of thought it would be cool if weakened from the surprise attack from the Wrathgate scene, Arthas instead was fleeing from us and spawning adds/mini-boss encounters along the way to protect him as he retreats into Icecrown. Either way, the idea of a chase scene would be awesome.

See that necropolis on the HoR map? My guess is that big scary flying things come out of it. Maybe Arthas lets you poke him for a bit (phase 1) then he says something in the lines of you becoming a great scourge luitenant since he's missing 2 atm, he stuns everyone and walks out, the party follows him (maybe on mounts) and then he ambushes the party with an entire army of scourge from the necropolis (phase 2). You have to survive wave after wave (set quotum or timed) and after that one of the 2 airships which are doing absolutely nothing in icecrown at this point fly in, shoots down the necropolis, aircrafts follow and shoot down the gargoyles and wyrms and tirion himself drops down and helps you defeat arthas at the end (phase 3). Arthas does QQ and teleports out. Tirion hands over chest with loot.

And for arthas favoured he steals Ashbringer from Tirion first, then sees Jaina/Sylvanas and sort of panics, teleports out and you get a chance of taking back ashbringer in the raid instance