I think this is something that has been long overdue. I think Frostmourne is obviously quite iconic, but given Arthas is effectively dead and gone, as is the weapon itself, I think it’s time to allow players to unlock it for transmog.
Personally, I would make it an “ultimate Death Knight reward”. Give players a secret, optional quest to speak to Darion Mograine in Acherus. He mentions that the Scourge seem to be growing restless, and there are reports of the Cult of the Damned attempting to usurp control over the Scourge.
While Frostmourne itself may have been destroyed, he believes that forging a facsimile of the legendary runeblade will keep any would-be warlords at bay — at least for the time being.
He tasks you with undergoing a number of challenges to try and harness the memory of Frostmourne. Giving essentially a “greatest hits” of tasks for Deatj Knights over the years.
I’d honestly maybe even just make it a basic checklist; forge Shadowmourne, get the Legion class hall mount, defeat the Mage Tower bosses as all three DK specs, acquire the DK ring from Anduin in Shadowlands, and get the new Frostmourne trinket.
Do all that, and you get a short little quest to forge your “replica” of Frostmourne. Maybe it’s completely powerless, or maybe it’s a legit Legendary in its own right.
I’d personally suggest having is talk to the Primus at some point, him perhaps even warning us that some small fragment of Arthas’ spirit may yet remain in the shards of Frostmourne (now the Blades of the Fallen Prince, presumably involved in creating the new weapon).
At the end of it, we get “Frostmourne” as either a transmog-only item, or as a proper Legendary, with an acknowledgement that while it may not possess the same power as the original, it will at least strike fear into any potential Scourge commanders for the time being.
Just reuse the updated Frostmourne model added in Legion, and maybe also unlock the old version from back in Wrath as well. Another near little flourish might be getting “glyphs” to add blue fire visuals to some abilities, but that’d just be icing on the cake.