It was announced a few days ago that Blizzard were finally allowing Enchanters to shatter 1 Maelstrom Crystal into 2 Heavenly Shards through Maelstrom Shatter.
As a result of this, based on the premise that on my server 1 Maelstrom Crystal is worth the same amount as 1 Heavenly Shard, I should imagine that in the short term (on the day, or possibly week, of 4.3's release) the price of Maelstrom Crystals may increase due to increased demand, but in the long term, Heavenly Shard values will decrease because the demand will stay the same whilst volume of them will increase. Consequently, there will be a short term rise, then fall for the price of Maelstrom Crystals (because in the long term they'll be tied to the price of Heavenly Shards) and a long term fall in the price of Heavenly Shards.
Therefore, if you are on a server with a similar price ratio to mine, I would recommend stock piling Maelstrom Crystals if you think you can shift them on the day (or possibly week) of 4.3's release, but not stockpiling Heavenly Shards.