This changes often because I’m still testing it. So take a peek every now and then. Also, I’m trying to highlight the main words for people to skip/read faster through it.
Summoning
Scale devil summon with magic attack and adjust if needed. The magic attack scaling could be effected by the summon skill.
Example:
Lv1-5 is (+15%) per point, Lv6 - 10 is (+20%) per point, and lv11 - 15 is (+25%) per point.
Lv1: 40%-----Lv6: 120%-----Lv11: 225%…
Lv2: 55%-----Lv7: 140%-----Lv12: 250%…
Lv3: 70%-----Lv8: 160%-----Lv13: 275%…
Lv4: 85%-----Lv9: 180%-----Lv14: 300%…
Lv5: 100%—Lv10: 200%—Lv15: 325%…
Then add an attribute that boosts devils power by 50% like other skills do.
Note: With card star at 10 (+50%) this makes a total of 425% of your magic attack (basically 4 auto attacks) given to the summoned devil. Riding factor % will probably need to be changed (lowered) from this.
Note2: The high % factor given to sorc will motivate people to pick Sorc circle 3.
Unlocked at Sorc C3. Add attribute to increase attack speed by 50%. Players in pvp can easily avoid melee cards. It’s not useful there.
Give summon cards double hit or make cards that hit two times only hit once. Increase melee cards movement speed by 30% - 35%. So they can get to the mobs faster to compete with ranged cards that shoot far.
Note3: Being able to use our skills while riding the devil would really help. We have to keep jumping off the devil to use all the other skills we unlocked and that makes it more complicated. Druid in wolf form gets to use all of their skills too, why not sorc too? 
Summon Familiar
Increase the Splash attribute to 50% - 75%. The current 5% doesn’t activate enough to even waste your money on.
Unlocked at Sorc C3. Add an attribute that turns your bats into summons! Now they do not kill themselves and will only attack enemies that are close. Each bat does 15% of your magic attack. When a bat dies or if its hp is very low, it will chase its attacker and explode, dealing 100% magic attack (1500% total on lv15). Having this attribute on takes the original skills 382% factor away and replaces it with the new one. This would be very cool! 
Evocation (Two ideas)
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Summons another devil.
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The new skill allows another devil to be summoned (from the secondary slot in book) for 15 secs with a cd of 30 - 35 secs. This devil gets a 100% power boost and uses its own skills. You can summon this devil a few feet away from yourself with the circle target. You now have two devils. Awesome right? T__T this would make C3 more worth it. There are many sorcerers that wish to have two devils out at the same time. If this does happen maybe make a hit box around the other devil so they don’t over lap each other.
Desmodus (Two ideas)
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Give an overheat of 2. Increase damage to 625%. The “blood sucking bats” now heals you 5% of the damage dealt to the enemy. Makes C3 more worth it. And reduce cooldown to 12 secs. (Fireball hits harder than desmodus by 52% and has a cooldown of 8 - 9 secs so why not desmodus too?) Also fireball can hit 2384% in 16-18 secs. This is not including sage dupe which doubles this again.
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Give overheat of 3. Leave factor the same %. Add the 5% heal when they hit. Reduce cooldown to 12 secs.
Summon Servant
Add an attribute to change the cat to a big dark spirit wolf that howls or a ghost girl dressed like a maid that sings a very short melody (lol). 
Maid could look like this.
Wolf could look similar to this.
The new skill is a buff that is activated right away. The buff gives summons more attack and defense for 2 minutes max. Something like 10% of magic attack and 10% of your own defense is added to the Devil/Bats. This skill also buffs allies with dark attack (SPR scaling). Your SP recovery will be cut in half while this is activated.
Summon Salamion (Two ideas)
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Scale this with magic attack too.
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Replace this with another skill that summons another devil from the book.
Morph
Combine both devil card stats into the second card slot that was picked. Keep Hp the same and keep physical and magical defense the same too. Add an aura or dark glow to the enhanced devil. Add and attribute that increases the morphed devils movement speed by 20% and reduce auto attack cd to 1 sec.
Thank you for reading! 
so waiting for this change 
And it would fit the “blood sucking” part in the description.