Hmm, they say the person that crafted the Shards was the Mage of Emerald ... (or something) ... yes?
In that case, how about having rare artifacts known as Gems of Emerald. Every Sovereign can start with one, or maybe have to spend a certain number of Sov points to start with one (5 sounds like a nice even number).
These Gems of Emerald are extremely powerful, and give a Channeler complete and full control of a magical shard for a split second. It is so quick, in fact, that although the Sovereign could theoretically control enough magic to tranform the entire world of physics, there are simply not enough Gems of Emerald in the world to let the control last long enough (hours) to actually destroy the nation or (days) to actually destroy/recreate the entire planet at once or (weeks) to recreate the laws of physics. Instead, you only have enough time to envision a single element. This knowledge has been passed along the lands, since the Shards were first created. At this point all the Sovereigns know of them, and of their function. Some say the Gems were created by the Mage of Emerald, others say they are pieces of her body. The world may never know. All we know is that it allows us a one-time shot to permanently change the element of a shard.
Use of a Gem of Emerald would allow you to change any one shard under your control to any element you desire. (however Kingdoms cannot use them to create Death Shards, and Empires cannot use them to create Life Shards).
The use of two Gems of Emerald could be used to destroy a Shard, forever losing its magical potency. (Perhaps giving you Essence)
I suggest all Canon Sovereigns (and default custom) start with owning a Gem of Emerald by default. It is something they can "remove" if they want an extra 5 points to spend elsewhere (similar to how stats start at 10, and you can turn to 0 to get 10 more points).
In this way, there is more than one obvious use, and its effects are as permanent as they are powerful. It would not be wise to underestimate the Gems of Emerald.
(Gems should be hard to find -at least guarded by a MegaBoss- and rare as well) perhaps 3-5 hidden in a standard sized map.
So then, you can Specialize in only one element, and keep the Gem, or you can "lose the Gem" and dish an extra 10 points to gobble up all 4 elemental books of magic.