Whether it's reforging Frostmourne (since we literally already have the shards), or forge a new one entirely, I just feel like it would be a glaring oversight NOT to add such a quest for DK's. Stuff like that and the Warlocks' Green-Fire quest are the kind of things that get players EXCITED about the game!
It seems so weird that there hasn't been any indication of creating a Mourneblade of any kind, though.