Summoning Daedra which continually eat your magicka in order to stay bound to Mundus seems like a relatively safe idea. It doesn't have to consume a whole lot, but enough that Mages can't simply summon their most powerful creature and continually blast their enemies with their most powerful spells at the same time. In my opinion, that's abusing the system. The same goes for warriors that have just enough magicka to summon a decent Daedra, summon it, and start knocking around their enemies with their biggest, baddest weapons.
A system like that would also be able to scale with the amount of magicka you have (rewarding those with higher magicka), the higher the alchemy skill (more magicka through potions/other means), the more you explore (assuming there are items that also restore your magicka in-game), and the more money you acquire (to be able to purchase any item/potion/etc.. that restores magicka). It could also use conjuration as a means to lower the amount of magicka consumed (or how often it is consumed).
It just seems a lot better than giving us static conjuration spells that work the same, regardless of your magicka and conjuration ability. And, for those warriors who like to have a decent conjuration skill, a perk could be added that allowed for summons to stick around while only cancelling your magicka regeneration -- not consuming (meaning they'd be permanent summons).