I am heavily stealing here from a certain work of fantasy fiction. Just so you know. I also have mentioned it a few times, but only here and there.
In the Vlad Taltos empire everybody can do sorcery by linking to the Great Orb. The Great Orb is a major (seriously major) artefact created by the empress. People still have to learn sorcery, but they can tap into the orb for power. Most use it for simple things like cleaning the street.
I would suggest a similar system. Every unit, Sovereign or Champion has a certain mana pool and a certain level of casting. A unit with 1 level of casting can use one mana point every round - so it might take a bit to cast a fireball and would be much better off hacking away with a big axe. And without mana they can't cast at all.
The Sovereign can create a Major Orb in his Capital (thus making his capital actually rather important). This gives every unit access to the global pool. All the tapped shards go into the global pool, unless the Sovereign has made a personal link to them.
The Orb has a limitless range, but the power provided fades out over distance. Thus, a unit drawing say 100 mana points far, far away in a foreign kingdom would only receive maybe ten (but 100 would be used up).
The Sovereign has, of course, the possibility of building smaller power towers, that reduced mana loss (the unit would still only get ten, but would lose only 10, instead of 90) or increased flow (the unit actually gets 50 instead of ten) or both.
The Sovereign or a Champion can dedicate personal mana for city spells - I am still hoping for a Floating City spell, or at least Floating Castle. The Upkeep would then be taken directly from their own mana pool. Or they could dedicate the mana pool for the upkeep.
If the Champion or the Sovereign died or ran out of mana the spells would fade.
Of course, why should a Champion ever use his or her own mana ? Here we come to the fun part. If the shards and temples are linked to the main orb, then by taking out the main orb in some way you could instantly destabilize the global mana pool. If the city was put into a Time Stop Bubble (yes, I am still hoping), or was covered by an Anti-Magic Cloud (still hoping) then things could become difficult for the Global Pool.
Also, if you imagine (or see) ley lines from the shards and temples to the orb you (or the enemy) can deduce the location of your capital (or just follow them there) and might be able to cut those lines, especially if they cross his own territory.
As a further tech to be researched (quite possibly civics or magic) is the amount of mana ordinary people can use and the spells they are allowed. If everybody is skilled at tapping into your power, then a simple peasant with a stick becomes quite a threat when the first fireballs are flying.
Empire Troops with Drain Health spells could reboost themselves by draining their enemies health, making mediocre axemen into dangerous vampire warriors. All, of course, on your power.
I think this would be significantly more interesting and permit more complexity to the game than a simple mana pool.
