Not sure I like general mana pool concept. I think current way works fine

Probably a better way to look at it is that shards will provide N mana.

Spells, therefore, will cost M mana and come from a global mana pool.

Because mana is global, strategic spells no longer need a caster but can simply be "cast" on the map which gives rise to all kinds of things that just didn't make sense when the caster had to be physically located nearby.

How does leveling up choosing "essence" work for both imbued champions and sovereign? I assume with "global" mana there is no mana limit for any unit, so for example if 120 "global man" exists in the pool then one unit can cast firestorm (6 mana spell) exaclty 20 times before the "global mana" runs out, and at at that point no other spellcasters can use magic unitil it regererates at whatever rate? If shards are the only thing that determine global mana regeneration, then as soon as you lose your starting shard (or AI loses theirs) you're/they're SOL until they recover or find a new one?

If you're limiting mana regeneration to shards that's going to create a huge bottleneck & every single human player is going to do everything they can to take these strategically important resources from everyong else right from the getgo .. it seems to me there won't be a need to take any AI city but the one with the shard, then do the same for the next AI & so on. So, if you're at war with 9 AI all you have to do is take over the shard and you've basically won? If this is how it's going to work, you need to add something to the fortification techs that allow units positioned at shards to "fortify" stronger than the current system (to give the AI a fighting chance, also shards being a magnet for wandering monsters would'nt be a bad idea either), otherwise the AI will be a real pushover. I would would have some kind of arbitrary defense bonus for AI cities with shards. Perhaps not showing (some) shards until high level adventure tech is researched will buy the AI additional time before the human "game start blitz" takes their shard cities out, cuz you know this it what'll happen .. in fact having a "shard search" adventure tech that is needed to reveal all shards other than the starting one will force players to research this tech before being able to search effectively for the shard cities, again buying the AI a little more time to beef up its defences. 

So does this mean "Essence" for he everyone other than the Sov will just be "yes" or "no"?  Or will champions have a certain amount that they can channel from the global pool (that they can raise on level-up)?

This is exactly how it should work. The Sov and each champion who can cast spells should have essence or whatever, a local pool they can use per turn, which ultimately draws from the global mana pool, they can use it overland or in tactical combat. Each turn it should either completely replenish (like MoM) or regenerate off some other statistic or something, but it needs to be more than 1 per turn and it really should be more than a static value. This would be great!

I don't personally see the point in making essence a boolean for any unit, all that would appear to mean is yes/no as to whether or not they can cast spells, while essence = 0, would mean the same thing in the above situation.

Or add a twist to it.
Good old D&D had a limit to how many spells a caster could "learn".

If you didn't have 18 INT, you wouldn't be casting lvl 9 spells.
Generally that wasn't an immediate concern because those were things like Meteor Storm - generally things that would register on the Richter scale.

What if channelers had "spell slots" for tactical spells?

Say you have 20 INT.
Putting a lvl 7 spell onto your active spells might cost 3 or 4 points. A low level spell much less.

That adds strategy to magic and an argiment for having more than one channeler.
Sure the champion imbueing costs upkeep but it adds flexibility because you're bringing more options to the fight.

And increasing INT would have a tangible benefit beyond the trivial damage bonus you currently get.
Damn. more choices...

Also:  "ready" spell slots would be a far better way to deal with magic in multiplayer tactical battles.
Leafing through the spell book for every spell is definitely not a good idea.

I am more about limiting the types of magik books you can learn.  If you have a water shard, then that's what you start with and the like. Some casters would have the ability to know two books.

To learn more magic, you'd need an event like some old wisend survivor from the titan wars apears and he teaches you, or you learn it at a much later stage in the research, even a super quest.

SO you ask, "why would I want to learn another magic book so late in the game?", because some spells require mutiple disciplines to lean them and cast them. So when you have two book like earth and fire, you can now create a volcano. Or the life and death book, and you can now summon a death reaper.  Fill in the blanks as to what should be there with mutiple books. 

That's what I call flavor.  By the way, I absolutely loved the random events in GC2, and in Civ 4.  The more the merrier.

So much for my fire/water giant rush.

While the idea of global mana (like a resource if I understand it) is fine, I still say that the system described:

1) rewards rushing too much. Since shards are so important, and everyone has one, it's almost a no brainer to grab an enemy capital or two as soon as you can. Once you do the game becomes super easy since you just became twice or more times as strong as everyone else. Twice the global spells, twice the combat damage, twice the uber summons... however you play, it is a no brainer.

2) it makes imbueing almost pointless. Right now I imbue to get extra mana regen. If imbue didn't do that, I would just stick with my sovereign as the caster and toss most of the heroes to the wolves. This basically mean that 8/10 of the heroes currently in game are made useless in combat (I'm talking about the ton of INT heroes). Only the str ones (mostly adventurers) will be used in battle, and with a bow. So in the end you are basically looking at a situation where you hire heroes to sit indefinitely in a city generating x points for the rest of their lives.

My suggestion:

Each faction naturally generates a certain base amount of mana (use the race config to do this). This can be seen as some innate magical power that is in all people, but they can't use it directly. The sovereign, being a channeler, can harness this energy and generates some mana. The sovereign himself should also generate some mana. This keeps the sovereign central and establish a "base mana" regen that you would need but completely remove the rush reward that I explained earlier with shard spawn.

Instead, shards should be rare, and spawned away from civilization (x tile away from all starting point depending on map size). They should still be powerful mana batteries (to encourage people to seek them out), but they should also have some strong guardian units (elemental army) that you have to defeat in order to claim it. You can easily do this through the quest system: you step on a start location (infested shard), beat the guardian, the reward is the actual shard spawns, and then you can build on it like a normal shard. This makes you build up some proper army before becoming super powerful through shard rushing.

Imbue should just cost mana, seriously. If mana is so rare, all you have to do is make the cost high and you'll find people having to make the decision whether to imbue more or save to cast spells. Say you have 20 mana, regens 2 per turn, and Imbue cost 10 mana. You can technically imbue 2 heroes, and 1 more every 5 turn afterward, but then you would never have any mana to cast a single spell. With the addition of more spell choices, and thus more things you want to spend mana on, this becomes a really bad tactic. Instead, you'd probably want 1 or 2 imbued heroes at most, and save your mana for other things. This keeps imbueing a few casters a valid part of the game.

It sounds like heroes are going to cast directly from the global mana pool which is exactly what this game doesn't need.

Basically you'll have a whole bunch of heroes as mini-sovereigns and never need to risk your sovereign again. Using the sovereign needs to remain a significantly better magic advantage than heroes.

If imbuing too many of them becomes an issue I'll just single shot them out each time one dies.

Also, making shards common is going to destroy their special ness and make them just another generic commodity.

Remind me again what is the story vision for Magic in this game?

I really can't believe you're still completely revising mechanics because of issues that arise that are predicted by the beta testing community each time. Please Stardock quit trying to reinvent the wheel because it isn't going well, copy the MoM system of Global resources if making the whole game a mess of RTS global resources. They also handled mana regeneration rather well with buildings and nodes, yet you didn't have to start with a generic universal node, the nodes were quite special and acquiring even one was significant unlike Elemental.

Look at how MoM handled magic, what is the advantage that you are proposing from their system and the differences you're proposing?

you might also do better with mana regenerating with a non linear formula. if you use

mana = max mana x (1 - e ^ (- game constant x turn number))

this chart shows mana regen over turns on the x axis, with a max mana of 20 (red) and 15 (yellow)

then your mana regenerates at a decreasing rate until it reaches your max mana. every summon or channeler reduces your max mana by one or two points and every shard increases it. the lower your max mana, the lower the regeneration. this means that you don't get rewarded too much for sitting around forever and piling up mana, you can have a decent number of summons and you still have decent mana regen rates when you've been using it a lot. plus it's impossible for the player to end up with negative regen.

crucially it means there is only one number for the player to deal with: their max mana. it's this that is increased by shards, reduced by summons and other maintenance, and determines your regen rate as well. everyone's mana pool theoretically takes the same time to refill from 0 to their personal max, and this time is determined by the game constant. this can very easily be related to the game speed option and tweaked if people think mana is generally unbalanced.

maths solves everything.