Excellent Thread. Got me to sign up to this forum.

Though I have only played with the archer so far, I will try to suggest some skills for the other characters.

Ok, here are some ideas:

Archer:
Multiple shot - Indeed a good idea.

Fast reload - A skill that would give the archer some kind of steroids and reduce the reload time from 1.2 seconds to 0.3, and he would be able to shoot like a machine gun.

Fire/Lighting/Cold Damage added to a shot. Instead of all those support skills like mentioned here above. It would also allow to develop 3 types of archers by the element skill they choose. Poison is also a great option, Maybe even a must have option.

Rain of arrows - fill a certain radius with arrows falling from the sky (the archer animation can look like he's shooting 100 arrows in 2 seconds towards the sky). would be an excellent collateral damage.

Instead of that "Vines" skill, which immobilize an enemy, I'd say it would look much better if it was just a plain old Paralyze Arrow. Why the hell would a Hawk Archer could summon vines from nowhere?

1 Shot Bulls eye - Maybe an exaggeration, But it would be awesome to be able to kill non-unique monsters with 1 shot. This one will have a long cool down time, and every skill level would decrease that time.
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Enchantress:

Confuse - 1 enemy of choosing will become confused and will attack other monsters. this skill can be either "Confused" or some kind of "Conversion" to the player's side. If we want to improve this skill, the monster will actually become the enchantress's body guard for some period of time, but then the skill will have success chances of working which will improve as the skill level is higher.

Target - Unlike "confuse", the "Target" skill will let the player choose a monster and will cause all other monsters to attack it. Not a must have skill in this game, but a nice add on in my opinion.

Raise Skeletons - As suggested here, Will be an amazing skill if implemented correctly. Of course, it doesn't have to be a skeleton. Can be a golem, a living elemental (Randomly selected) and so on.

Clone - The player will be able to create an exact clone of his character that will fight with him. I'm not sure if this is possible with MMO's, but It's an awesome skill.

Firewall - Block the path with a firewall, preventing monsters from advancing towards the player and damages them as they try.

Leech Life/Mana - A buff which will let the enchantress to take monsters life or mana once they're dead. Can be also while they are alive, but I think it will have a much nicer touch if it works only when they're dead. Every monster within a certain radius that dies (doesn't have to be by the current player's hand, can be a party member who kills it), will give the enchantress a certain amount of life/mana.

That "Vine" thing that the archer have - can go to the enchantress. Makes a lot more sense.

Increase Luck - Kind of a nice gimmick more than a useful skill. Will increase chances of finding rare items, more gold, more potions or whatever.

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Warrior:

Sacrifice - A huge boost of strength in the cost of HP. Dangerous and effective altogether.

Charge - The character will charge towards the enemy, damages anything in it's path and deliver a strong blow to the target at the final destination. Will cause the warrior to be dizzy for 3 seconds or so. Needs a very good timing and skill from the player to do serious damage. Needs to be used wisely.

Battle Roar - A shout that will buff the player and the party with a boost to strength. Will be beautiful to see and experience in a big party before a mob fight. Another skill related is a roar which will scare the monsters away for about 5-10 seconds. useful when fighting a big mob.

Fortify Armor/Weapon - Will give the current armor/weapon a boost in stats and/or another property.

Thorny Skin - Kind of "reflect damage" skill. A monster that will attack the character will receive a certain percent of damage back to her. At the highest level of this skill, A monster can get 75% damage back. Will be very useful against much stronger monsters.
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That's all for now.
Cheers!