Here tell me how this sounds for a way to get permanent summons.
Perk: Rite of Summoning
Requires: 60 Willpower, 70 Magic and 75 Conjuration
Gain the ability to activate summoning ring in Unnamed Dungeon.
The summoning ring will be at the end of this dungeon and this will be a high-level unscaled dungeon (if those exist). The summoning ring will require a couple of full grand soul gems and it will be built around a destroyed Oblivion Gate.
EDIT:
A few things to add. First, Daedra only. Undead aren't generally that interesting and while I support them adding Necromantic options (maybe even a permanent summon perk that is mutually exclusive with this one), Daedra as conjured seems more fitting. Second when you activate it, the ring brings up a list of Daedra types you can make permanent and when you choose one it will select a random name for it. These bound Daedra will scale with you. Finally each time one of your bound Daedra is killed you can re-summon with more full grand soul gems and Daedra Artifact (or equivalent).
This I like. Perhaps in addition to the gems a sacrifice might be required for more powerful Daedra, but the sacrifice could be of a hostile NPC as easily as a non-hostile. Whether blood would be required, or a heart etc. It might be something different altoghether, like a lesser variant of the things which The Daedric Princes require before the offering of quests (Glow Dust, Troll Fat, Minotaur Horns, Ogre teeth etc.)
I also think, rather than constantly draining a players magicka whcih seems unfeasible, and which doesn't seem to be the way long term (permanent) counjured Daedra behave at the lairs of Conjourers, I think that the initial ritual should carry the greatest risk, and the success or failure of control from the onset should be largely based upon the skill of the caster. I.E. A journeyman who summons a Dremora, an Aureal/Golden Saint, or a Xivilia could achieve the summons but might be attacked on sight, whereas a Master would receive proper service from these creatures, (though, depending on type, more willfull, chaotic beings like Xivallia might rebel if the sorcerer consistently is disrepectful in speech or interaction with the Daedra. The likelihod of revolt should be less with Dremora and those other Daedra types who are used to operating in a hierarchal system, and are keen on loyalty etc. , whereas the Xivallai are infamous for betrayl and revolt in the face of disrespect). A lower level conjurer who summons a Daedric servant but is attacked by it will still gain in conjuration skill, provided he or she survives the attack. As an added penalty, the sorcerer may incur the permanent dislike of that Daedra, who may hunt him/her across Skyrim until banished or trapped.