#1. Add a special price to the Transmogging of Legendary items, one that involves current Raid or PvP content and that will always update with new content. (So that if players want to keep their Legendary items useful, they'll have to fight for it.
#2. Better yet, make it so that the Transmoggers or someone, can upgrade the Legendary items themselves, using current PvE or PvP content such as points or rare drops. This would allow the item itself, say The Hand of Sulfuras, to assume the stats of the most current PvP or PvE items but have the names and appearance of the originals.
Opinions are more than welcome but let's keep them civil and on topic please.
PS: Think of it this way, it's not very Legendary if it just coolects dust in your bank...