Well, flavour is important, but game mechanics are more so. There should be some restriction, and it is not too hard to come with explanations, e.g.:
- Elemental royalities simply do not exist out there on there own, but are distributed all over the world. The summoner forces these raw elemental power to form the entity it forms when angered. The spell's power to bind the raw elemental power into an AirQueen only last a number of turns, but can be renewed as well as stolen by an enemy mage. Otherwise the force slowly starts to disperse again (slow loss of HPs?) until it is called upon again to concentrate and form the identity it has done so a thousands of times before. Raw elemental powers like it quiet, until angered that is...
- Check common fairy tales why it might be a bad idea to bind devils to do your bidding. Devils need to be amused somehow, so an upkeep in the form of blood slaves (taken from lab) is not too unrealistic, is it? A lack of slaves must not drive them away, as devils take promises all too often, but their need could make it difficult to assemble enough blood slaves to summon even more. I am also happy with Devils/Demons consuming a quarter blood slave or less each...(e.g. as in once every four month)
- Undead and especially vampires feed on the living and spread diseases, so each undead in a province might yield a chance to tip the growth scale in that province towards death or even spread disease like some already do. This would not hurt the undead themes, but would make it more difficult for non-undead themes to use undead in abundance...
Furthermore, I did not understand it that way that Wraith Lords were given immortality by the mage, especially not by pretenders lacking immortality by themselves (a ritual to make
sacred commanders immortal would be nice). So why do they serve at all? Because of an old forgotten vow? No, thank you, I had enough of that before in that movie...
)
Again, the restrictions for national troops are plenty, thematic and useful, but summons are a bit out of the way. I am not saying though that ALL summons should require upkeep - wolves already need food, mechanical men need resources to heal, etc. - but a bit of variety cant hurt the game, can it?
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I agree on the experience issues, as I wrote in an earlier tread months ago, experience for national troops should be enhanced. Luckily, mindless troops do not get experience already...