The first one is a biggie for me at least but I guess it's too late unless already planned in secrecy.

THE COOLNESS FACTOR

When casting a global spell it should feel like you are doing something especially powerful, possibly world altering. This could be done by playing a simple cinematic sequence using the game engine. So even if you are all zoomed out on the cloth map you would see some more or less epic spellcasting.

E.g. first show a close up of your Sovereign, standing on the roof of his Sorcerers Tower, chanting and gesturing. Then cut to the effect. A volcano rising out of the ground, consisting of a close up of shaking ground and then a wide shot with the whole thing coming up.

The more powerful the spell, the longer the cutscene. Could be from 5 (simple unit enchantment) to 15 (volcano stuff) seconds long. With an option to turn them off for those who want the game to look and play like a chessboard.

OTHER THOUGHTS & SUGGESTIONS

• An Arcane school of Magic is missing. Think of Sorcery from MoM or AoW:SM. You can't possibly squeeze all spells into the Elemental schools or Life or Death, especially mind/force or countermagic spells. Specializing in that type of Magic should also be an option as controlling global spellcasting is exceptionally fun as is evident in Master of Magic.
• There should be some way of storing Mana. It could be a Mana Vault city upgrade or a Mana Orb artifact. But limiting Mana by Essence is just that.. limiting. Essence could still limit the amount of Mana usage per turn.
• Spell cooldowns are not a good mechanic. If I want to summon 5 giants in a row I should be able to if I have the Mana. If cooldowns will remain, there should be a way to make every spell more powerful e.g. summoning 5 giants with one spell for 3 times the mana cost. But generally, multi turn casting times would be better than cooldowns.
• Some kind of global spellcasting upgrade for a city. AoW:SM had Wizard's Towers you could upgrade to get more range for global spells and it was a very good mechanic. In Elemental, you would also have the choice of simply walking your Sovereign close enough to an enemy city to unleash some nasty plague, but how cool would it be to have a massive tower in the middle of a city with your Sovereign standing on top conjuring something nasty. Think of Sauron's all seeing eye etc... a tower for casting would be very cool.
• Spell research needs to be fixed so it's not possible to waste spell points. Perhaps just choose spells in advance like in MoM. It makes more sense to choose what to research first anyway.
• You should be able to specialize deeper in a school of magic at Sovereign creation. Perhaps able to pick a maximum of three books of the same school to get more starting spells as well as more late game potential in that school. This would help a lot in making the Sovereigns more distinct.
• I'm wishing for more minor summons that would not require a shard. Salamander for Fire, Thunderbird for Air, some twisted angry Ent for Earth and a frost breathing beast for Ice. But even the first summons shouldn't be too weak and they should always be more special than some man at arms you can produce in any city. Who would want to summon some weak basic creatures when you can train those.
• A high end specialization feat for Sovereigns to become a living shard. Perhaps if you chose to fully specialize in a school of magic at Sovereign creation?
• Ice is a much cooler name for a school than Water. Literally.