Ever since the Shadowlands and the cutscene where Uther’s soul was split in two by Frostmourne it has always left open a possible idea that Arthas survived, or rather a part of him in the some manner. A fragment or such.
What if when he claimed Frostmourne his soul or being was split in 2, one part remained the other taken/removed/or separated. There was the theory that a part of the good that was Arthas first survived within his own heart.
If you recall the events of the Icecrown Quest Chain, you come across the heart in the deeps beneath the Icecrown Citadel and a ghost or apparition appears in the form of Matthias Lehner. Believed to be the last spark of his humanity.
Tirion only saw “shadows of the past remain” and destroyed it with Ashbringer wounding the Lich King. But given the nature of the object and its connection, what if the spiritual piece of Arthas still survived.
Given the nature of what we saw with Uther first, and then with Sylvanas, how their evil natures were in control first but only after reclaiming the other pieces were they made whole. Sylvannas good nature could not believe what she had become but was forced to accept it.
Now flash back to the events with Anduin and the Mourneblade he possessed. Using the fragment of Arthas soul that was collected when he was killed and tossed into the maw, the evil nature of Arthas disappeared upon the shattering of the blade into its 2 respected halves, something Zovaal knew nothing of.
So what remains or could remain of his other half? What if it merged with another soul or being? Could Arthas return in some form or conscious thought of another host?
Personally I’ve always believed that Arthas was wronged and should return but that is my belief. Not sure how others would feel.
Spoiler Idea
I thought of making a character that is host to the fragment that survived and guides a new life/paladin to redeem the mistakes of the past. Rasath Neltheim (anagram for Arthas Menethil).
I figured in some manner the split would have started (like split personalities) after the events of Stratholme. Killing so many could have been the beginning and grasping frostmourne sealed the split.