THIS MOD IS NOW OBSOLETE (KIND OF)
I made another one who adresses the same problem but in a more complete way.
No More "too powerful" messages

JANUARY 2017 UPDATE
I'm pleased to announce that the high mana cost of dread zombie has been corrected.
The reanimation spells now cost the same as vanilla.

Ever played a conjurer ?
Annoyed that the reanimation spells FAIL all the time ?
This is due to the poor design of the vanilla spells, the maximum target level cap is so low they quickly become unusable.

6-13-21-30 max level of a raised creature (7-16-26-37 with necromage perk)
6-13-21-30-40 max level of a raised NPC (7-16-26-37-50 with necromage perk)

What this mod does (max target level without necromage perk) :
raise zombie 6->9
reanimate corpse 13->22
revenant 21->34
dread zombie 30->252
dead thrall 40->252
ritual stone power 75->252

Why ? :
- If you use a "high level enemies" mod it breaks the whole conjuration, your minions are so low level they get killed in 1-shot and do no damage.
- Perks overhaul raise the maximum level cap by adding a % to the spell but they are too tame, I haven't seen one that goes above creature 60-npc 80
- This mod is very basic and very compatible, it only changes 2 things, the maximum npc level value and gives the spells a fixed mana cost
- Values are based on the level at which you can loot the spells in vanilla (raise zombie works till you start looting reanimate corpse at lv10) 
- 252 has been chosen because that's the player level required to unlock all perks in vanilla, I doubt many players go above that.
- Dead thrall and dread zombie use the same value because they are different spells, they don't get the same bonuses from perks or buffs, don't follow the same game rules and also dread zombie has the "turn to ash" fail safe to prevent you from doing dumb things like bringing a giant or a mammoth into your home, that's why dead thrall is and should be kept a NPC-only spell and why we need both in the game.

What this mod does not :
change the way the spells work
change the Vampire's Servant spell -> would conflict with vampires overhauls
change atronachs -> they suffer from high level scaling too but most perks overhaul fix that already (or try...)

Optional :
I uploaded an "unlimited" version that raise all reanimation spells level cap to 252.
It's meant for players that already are high level.
It will fix an invisible but real problem, vanilla NPCs don't have access to dread zombie, vanilla Serana can only raise level 21 mobs with her revenant spell by the time she is a follower I'm pretty sure lv21 encounters will be a thing of the past for you.
Several NPC enhancing mods also give the dread zombie spell to NPCs but they don't change the spell itself, making your lvl 125 foes unable to raise anything above level 30 :/

Requirements :
- Dawnguard
It's not actually required but my mod carries USLEEP changes fyi

Compatibility :
- Compatible with most perk overhauls like Ordinator, Perkus Maximus, SPERG
- Semi-compatible with mods that change standing stones (load my mod 1st and let your other mod override my Ritual change)
- Incompatible with complete overhauls like Skyre & Requiem, they do several changes to how reanimation works, don't even try.

thank hailweiss for reporting the very high magicka cost problem, the fixed has been fixed ;)