Hi,
For me mages are weak in armour and die in 1-2 hits. To counter their weakness, they output high damage whist also boasting mobility and singular/area of effect spells with creative defensive abilities, there seem to be a lot of other classes in this game so I'll omit crowd control abilities.
These are my favourite spells from past MMORPG experience.
Area of effect (AOE) spells
Evil spirits - an AOE spell that casts ghosts that run around and inflicts damage to anyone within the area (it also spins them around causing a crowd control effect). https://www.youtube.com/watch?v=ifKQra0X3fA
Holy shock aura - turn on an aura shock passive skill that consumes mana whilst on, any enemies that enter the players aura (eg 10 meters) are randomly shocked (every 3-10 seconds) with random lightning damage between 200 damage to 500 damage.
Defensive spells
Mana shield - using mana to absorb incoming damage
Teleportation - teleporting to a distance in front of you, this should also allow you the ability to teleport through obstacles such as walls
Super speed - increases players movement speed by 100% for X amount of seconds
Force wall - creates a solid force wall (the player can choose where to place the force wall) that can absorb a certain amount of damage. The force wall is large and therefor can protect other players. Additionally, the force wall can not be solid and instead enemies can walk through it, however they will receive large amount of burn damage - additionally if they stand on the force wall they will take the burn damage consistently
Single spells
Fireball - pretty standard single attack, however... launching a fireball - would be good if this would explode into bursts of fire upon impact of the enemy, leaving fire on the ground and if enemies walk over the fire they receive burn damage for X amount of time
That's all I can really think of at the moment!
Cheers,