One of my favorite things about Master of Magic was that it had spell and unit upkeep, I'm not sure if this game has unit upkeep but I'm pretty sure it doesn't have spell/magic upkeep(or I have not seen it yet). I'm able to cast spells, specifically persistant things (like enchantments, summons, etc) and they don't appear to drain my mana, once I cast it, I don't have to pay for it, I don't really care for this as it cheapens the game imo.
I don't have much of a reason to expand my borders in this game, other than having additional cities where I can train additional units (at the same time) or bring more resources to the economy. There is some impetus but not as much as their could be. Upkeep costs solve this problem because if I had to pay upkeep for my enchantments and summons and anything persistent really, I would have a reason to spread out and take over more area to increase my reserves and take over shards, not just to 'double magical effects' but to support more enchantments, summons, units etc.
Right now it feels like, an ever expanding source of power and resources and that is just not realistic and actually lowers the value of things to me. I can fund and create super powerful units with enchantments (my sovereign) and so there's very little reason not to do so, what's stopping me? It's funner to pluses and minuses to various decisions and I don't feel like there's any reason not to stack up the enchantments and summon as much as possible, Im' not penalized for doing so, so why not, but it's dull and doesn't 'feel' powerful. If I want to grow stronger I should have to go and grab more sources of that power, not just grab a little and run to the end of time with it continuously granting me ever growing power.
Anyway I liked that about MoM, it made magic feel powerful because you had to support it and you had to decide which enchantments and spells were worth casting and maintaining in various situations at various times. No wizard should just know all spells and be able to cast everything at will, etc.
Additional, if you wanted to have more spells and summoned units in play, you had to be able to support them, so you naturally, generally, wanted to expand out and take over more magical nodes to increase your overall magical power. With this game, the way it currently is, it's Essence and I can cast a spell, I do have to wait a while before I can cast again (that's kinda cool, though it's a bit debilitating with how slow it is right now), but given a long enough timeline, nothing stops me from summoning an army of fire giants or whatever, upkeep should keep things within check.
Upkeep is a good thing 