Having tried to read through much of this thread, and some of the others, here are my thoughts on the proposed GMP system - and if any ideas are duplicated from what someone else has already thrown out, then simply take that part of my opinion to be an agreement with your prior suggestion.
Anyway, here are my basic thoughts:
Whenever a caster, or party containing one, is selected, amidst the UI panel at the bottom would include a horizontal mana gauge (much like in combat); the same to be found within the Kingdom Summary page.
In both cases, if any upkeep-requiring imbued persons, summons, or other things are active, there would be a vertical line denoting the now-maximum mana that can be accumulated/re-generated with that % subtracted out. This would help you to see, at a glance, how much of your total mana is going toward maintaining all of the things that you've summoned, etc.; otherwise, those things could easily get away from you.
Next, if you [left-]click on the gauge, it would bring up a new screen, listing, in detail, the different items/creatures/persons/spells consuming the mana upkeep. From there, there would be an X right next to each one whereby you could simply click once and release the mana by canceling the spell cast. This introduces the additional ability to un-summon a creature (which may already exist, but I have not yet found), but in any event, allows for quick-and-easy management of your global mana.
Along those same lines, strategic spells should be modified to also require permanent upkeep, and thereby reduce your overall global mana pool. The sovereign should have to choose a balance between maintaining additional defenses/resources within their towns, and how much of their magicks they wish to devote toward conquering the world. So, for example, they can go all-out and spend everything on massive tactical combat, and hope for the best with the towns & families that they have left back home, or, if they opt for the diplomatic route, spending their time building strong relations with their neighbors, then they would need less mana reserves for combat and could use the rest to bolster their towns' resources, defenses against monsters, et. al.
Lastly on this point, mana should absolutely be shared between channeler, imbued heroes, and the like. Now, I could see children expanding the pool because of their inherent magical nature, as with the sovereign, though the advantage of having imbued champions able to conduct tactical combat beyond the reach of a lone sovereign is what you are paying for in upkeep; that, and the ability to have your sovereign out of harm's way. The alternative is what you have now: imbuing a bunch of champions, and then each runs around the board killing everything at once, without regard for how much mana is spent in any one battle, because you have four more champions with three more chain lightning spells up their sleeves with which to clear that half of the world on the same turn; all the while, the sov. sits safely at home, behind stone walls.
Adding a magical attack bonus, of sorts, to the sovereign, as others have suggested, might make for a good lure to bring that sov. out of hiding….
Additionally, whomever mentioned that starting with a fertile land, and a gold node, and a shard is a lot of give-me's right out of the gate, I agree completely. If it requires so many things to get off-the-ground with, then maybe it's just too much. Besides, shards should be a boon, not a requirement, and racing to beat other kingdoms/empires in hunting down shards is most definitely a worthwhile strategic pursuit; but, possibly having other nodes which bolster units against magic could be an equally valuable alternative.
Say, instead, that your early units only require food, and one other resource, but that there were canned options for troops that could be made from one of four to five of those "other" resources? For example, a phalanx requiring bronze (which would need to be added to the game, of course), a spearman fashioned with wood, brigand (dagger-wielder) made from iron, minor mage made from crystal. Any is a possibility, and it would be coded that you would start with *one* of those required resources to get you going, but each time could be different, introducing a bit more variety at the beginning.
Now, you could easily balance the "starter" units by having the weaker ones require less of their given resource, so that more can be built in the same amount of time, but then initial combat strategies would be a bit different based on the units that you could make.
Long-term, of course, you would still expand out, seeking other resource nodes with which to fashion additional unit types; plus, having more resource types can bring about new custom unit designs, such as "Hunter" units with their own minor version of an animal familiar (resource utilization = food, wood, animal [could be added with wandering animal clusters available to be hunted/tamed] ).
Did I go too far with that last one? Perhaps.
I guess the long-and-short of my mental meanderings here is that I like what Stardock is doing; introducing more options for strategy is definitely the right way to go. Keep up the good work, and thanks for your time.