According to this post, a new tier of charms called Mogu Rune of Fate will be used in the 5.2 raid. You trade for them with Lesser Charms of Good Fortune, just as you with Elder Charms. It hasn't been revealed how many Lesser Charms are needed to exchange for 3 Mogu Runes. Hopefully it'll be 90 or less. So if you're done with this tier of content, start saving up those Lesser Charms. Lesser Charm of Good Fortune is capped at 1,000,000.