I think Xur should sell sell Gjallarhorn one last time so all the people who have been day one players have the guaranteed chance to have one before the end of Year 1 Destiny. Much like Icebreaker, Gjallerhorn would only be sold once more, this would give Veterans who don't have Gjallerhorn a little bit more than just a shader and a sparrow. With the new batch of players ready to come in the Taken King, they would not have the easy chance of obtaining this powerful weapon out of the RNG system, letting Gjallerhorn's rarity not overall decrease since most newer guardian coming out for the Taken King will not have it.