If mana becomes a global pool and essence is a binary value, what implications would you like to see that have for gameplay?
Does this mean that every caster with essence will be able to cast every spell? What role will wisdom play? Will there still be only one type of mana? What will effect will shards have now?
I'd like to see wisdom become the amount of mana a caster can channel per turn. Any character can cast any spell, but if you're casting a big spell with a low-wisdom caster, it might take multiple turns to cast. In tactical combat, wisdom would determine mana channeled per AP. Because some big spells would take multiple strategic or tactical turns to cast, the magical system would now have a use for spells allowing you to detect other casters' current spell and interrupt spells.
I'd also like to see shards decrease the mana cost of spells in their "class" by a set number per shard owned, rather than the current damage multiplier. I think that allows them a role in all magic, rather than just tactical combat damage spells, and also makes spells more easily balanced.
It would also be nice if arcane knowledge were removed and replaced with a mana cost to research spells. The mana slider system in Master of Magic was great because you had to make important choices about how much mana you were going to save, versus how much you were going to use in research versus how much you were going to improve your casting ability. In Elemental, mana could be split between being used to increase the size of the mana pool versus spell research versus imbuing versus use in casting, giving the player important and interesting choices about resource usage.
How will mana be generated? Each caster with essence should add a small amount of mana to the pool each turn. Controlled shards should add a significant amount of mana to the pool every turn, since they're the source of all magic in the world.
Other options would include giving each caster a spellbook a la HOMM. Then spells could be obtained by visiting cities with spell training buildings and rare spells could be learned by individual casters via quests, thus making a real difference between different spellcasters. Wisdom might also be used to determine how fast a caster could learn a spell or whether they could learn the spell at all.
What implications would you like to see from a global mana pool and binary essence?