Does it have to be a Sword that is no longer in use? Could it possibly be that old knife Harry picked up in his latest romp?
Other than cuz I like the story I made?
Mostly because it fits it two ways, IF Raphael is 'out of the game' and the angels do have some connection to the Swords, then it would make sense for it to be a physical representation of his 'absence'.
Granted that still falls into the WAG mentality of reasoning, so the other argument is based on the other relics and what event they are pointing to...Spear of Longinus makes more sense.