It's a general rule in most games that ranged characters are impossible to balance against melee characters. Mostly, ranged characters (like mages and rangers) are a lot more powerful than melee classes; which is of course bad if you're planning on pvp'ing. A pvp/pve game is never balanced, because you have to choose. If you make a mage as difficult to play as a melee type in pvp, the mage will be useless in pve where you get zerged and will have to get hit at some point. The mage is a glass cannon character, which means it's defensively worthless, but offensively brilliant. It's part of why I dislike magic in games; because when you're melee (and dont have a party) and come up against a mage that knows what he/she is doing, you're dead; simple as that. It's 'imba' as they say.
So the choice you have to make is; will you make intelligence affect only the mana pool upon which to draw, or will you let it affect mana pool and spell efficiency and dps rating, or will you create another attribute for spell efficiency and dps, and keep intelligence for mana pool? Personally, I think mana pools is one of those concepts that RPGs should find a replacement for... something more like the D&D games, perhaps; but most importantly, something that stops the mage being a glass cannon, reducing its damage significantly; but also encouraging the mage to spend his/her points in vitality and dexterity, increasing its survivability.
I would much less dislike magic in a game if the mages were gifted people who would fight with staves, perhaps, and once in a while, when necessity requires it; cast one of their special abilities; making mages more like clerics and less like the end all of DPS. That way you could also explain magic in the world far better, rather than something intelligent people just magically attain, and a warrior HAS to be stupid, etc. etc.
I have long dreamed of a game which has a credible, perhaps low-fantasy explanation of mages. Something like sages; loremasters who can give the best insights on many issues, that know which herbs you need for healing and know best how to properly apply them; that can tell you on beforehand which is the best way to beat a certain foe, and so on. Someone who can't one-shot foes or just whip out a ridiculous fireball to kill everyone around him; but someone who is a gifted stickfighter and can offer many special insights into matters (such as dialogue options) which other classes can not. Which would end the mage's reputation of a glass cannon manafarmer, and would create a whole new image of mages, not butchering the wisdom of such a character; but reducing the balance issues and ending the whole 'run and cast, run and cast' type of exploits that come with mages.
Please don't shoot me for my opinion, but the everpresence of incredible and over-the-top magicks including the whole manapool and spells out the bunghole tends to really decrease my immersion of the game. I'm sure it's cool in outer space; but for a medieval fantasy world I just think it's distasteful. It strongly reduces the amount of feeling you can get for the story and the character. I think it's boring if you can just say "Manus Dei" and everyone around you dies; then quaff a few mana potions and rinse and repeat.
I feel the same way about health potions and such as well, by the way. Lose the whole potion idea. Healing is a way you can make the mage invaluable without strengthening the mage cliché.
Look back at the origin of wizards and such. It was for a good purpose that the wizards in Tolkien's books weren't human; they were Maiar, immortal lesser gods, and I feel only they can possess such powers without it getting tacky or far-fetched.
