Ok, first off, All the credits goes to guy/girl called Apollinator. I'm not copying, I'm just posting my version of this wonderful Goddess you had in your mind.
LORE
The Medusa was an ugly creature. Let's have a look at how she came into existence, for she wasn't always that ugly... Again, the Gods played their role.
The Medusa was the daughter of Phorkys and Keto, the children of Gaea (Earth) and Oceanus (Ocean). She was one of the three sisters known as the Gorgons. The other two sisters were Sthenno and Euryale. Medusa was the only mortal out of the three.
She was originally a golden-haired and very beautiful maiden, who, as a priestess of Athena, was devoted to a life of celibacy; but, being wooed by Poseidon, whom she loved in return, she forgot her vows, and became united to him in marriage. For this offence she was punished by the goddess in a most terrible manner. Each wavy lock of the beautiful hair which had so charmed her husband, was changed into a venomous snake; her once gentle, love-inspiring eyes now became blood-shot, furious orbs, which excited fear and disgust in the mind of the beholder; whilst her former roseate hue and milk-white skin assumed a loathsome greenish tinge.
Seeing herself thus transformed into so repulsive an object, Medusa fled from her home, never to return. Wandering about, abhorred, dreaded, and shunned by all the world, she now developed into a character, worthy of her outward appearance. In her despair she fled to Africa, where, as she passed restlessly from place to place, infant snakes dropped from her hair, and thus, according to the belief of the ancients, that country became the hotbed of these venomous reptiles. With the curse of Athene upon her, she turned into stone whomsoever she gazed upon, till at last, after a life of nameless misery, deliverance came to her in the shape of death, at the hands of Perseus
Role - Mage, Possible Assassin.
Attack Type:: Ranged As an Mage. Melee As an Assassin
Magical Melee and Ranged.
Pros:
Extremely High Single-Target Damage
Doesnt have to afraid of dying because of Passive.
Cons:
Very high mana drain.
Sqyishy early and late game.
No escape nor gap closer
Starting Health: 370(+60)
Starting Mana: 310(+64)
Speed: 350(+0)
Range: 55(+0)
Attack/Sec: 0.83(+0.97%)
Damage: 37 + 1.70(+0.24%MagicalPower)
Progression: None
PhysicalProt: 12(+2.3)
MagicalProt: 21(0)
MP5: 7(+0.43)
HP5: 9(+0.39)
Passive - Death Sigh: After taking lethal amount of damage, Medusa unleashes an screech giving her the power of casting 1 spell (even when its on cooldown) in 5 seconds before dying completely. Otherwise, boosting her magical power by 3 every level stacking up-to 20.
Max Stacks: 20
Magical Power/Stack: 3/lvl
Ability #1 - Poison Pool: Medusa spits out 3 small pools what does damage over time (30/60/90/120/150) and applying an after buff which stays 4 seconds after leaving the pool dealing 5/10/15/20/25 damage every 0.5 seconds.
Attacks: 3
Pool Damage: (30/60/90/120/150)
Poison Damage: (5/10/15/20/25)
Pool Duration: 5 Seconds
Poison Duration: 4 Seconds
Mana Cost: 70/80/90/100/110
Cooldown: 15-15-15/14-14-14/13-13-13/12-12-12/11-11-11/10-10-10
Ability #2 - Toxic Waste: Medusa throws an vial of Toxic Waste in targeted location slowing for 1/1.5/2/2.5/3 and poisoning everyone inside the targeted area dealing 20/30/40/50/60 damage every second lasting 3 seconds.
Slow: (5%/7%/9%/11%/13%)
Slow Duration: (1/1.5/2/2.5/3)/sec
Damage: (20/30/40/50/60)/sec
Poison Duration: 3/sec
Mana Cost: (60/70/80/90/100)
Cooldown: (9/8/7/6/5)/sec
Ability #3 - Inner Beauty: Unable to move, Medusa heals herself 5%/10%/15%/20%/25% of her maximum health over time while being immune to abilities.
Heal: (5%/10%/15%/20%/25%)
Mana Cost: 60/70/80/90/100
Cooldown: 30
Ultimate - Stone Gaze: Casting an ability for 1 second, Medusa unleashes her Gaze, petrifies enemies in a cone and luring enemies around towards her.
Stun: 3/sec
Lure: 2/sec
Damage: (85/95/105/115/125)
Cast Time: 1/sec
Mana Cost: 230
Cooldown: 180/140/100
LOOK/SKIN
She should look like an young, beautiful women using white, long dress whit poison green snake pictures.
When attacking, she "spawns" an rod to attack enemies in afar floating in air and her hair-snakes gets alive moving around her head.
Conquest Strategy
As an Mage she always should go mid. In Early game, Poison Pools can easily get minion kills and harass enemy mid hero, but can always keep the ganks low for the high stack dmg the Poison Pools does. As an mid game, she should focus on separating enemy mid pushes whit her Toxic Waste and Poison Pools. Teamplays are more easy if you have hard AoE heroes. When you cast your Ultimate, you should be careful, it does stone/stun the enemies inside the circle around you, but it will make you much more vulnerable for heroes like Thanatos, Thor, Agni, Freya and almost all ranged heroes with hard single target dmg, so remember to use it right time when you are sure every1 will be in and your team ready to cast them all down.
In late game this god is squishy. Theres no defensive items for her or even a way to cast the spells/abilities that way. So you should always stay with the Support or ADC and help both of them out as much as you can.
The ability you wanna focus first is you Poison Pool for hard and wide damage, keeping the ganks low from behind and from jungle. next one is the Snake Nest. You can deploy them anywhere in range to do massive damage on enemies. Remember, the snakes moves much faster then the players and cant be killed. Use that as an advantage when going in a fight or trying to run from gank.
Skills order should be 1, 4, 2, 3 For maximum damage.
Items type you should get are much Magical Power and some Penetration. If enemy haves smart support character which goes high on magical defense, Penetration and Poison Pools can easily drop him down because all 3 pools stacks on each others.
Actives should be meditation if you surely think the game wont take longer than 10-15/min but if you are not sure go with Blink/Combat Blink to maximum your ultimates use, and Speed or Enfeebling curse to slow down your enemies or catch up whit them to get the extra kill and money.
What do you think? Remember, credits goes to Apollinator, the original guy/girl who had this idea.
This is just my version of it.
EDIT: I might rethink the Inner Beauty and change it somewhere more stunning or dmging.