...A Mage who actually does what a Mage is supposed to do: cast spells.
In Tomb of Ice of Descent 1st Edition, there was a mechanic I liked a lot: feats. Each time the heroes found a Treasure, they would draw a feat, a card they kept hidden from the OL that could mess with his plans. The most powerful one was the feat that countered an OL card.
A spellcaster would need a deck of spells. But he should have access only to very few spells that are not very powerful at the beginning; the most powerful spells should cost the most XP. And once a spell has been cast, it should be gone for the rest of the encounter (or at least until the whole deck is reshuffled), like in the Advanced Dungeons & Dragons of old.
It would be something like: the Spellcaster class.
In addition to his skill cards, he has a deck of Spells that he is able to build during the course of the campaign by spending his XP. Whenever he draws one, he keeps it hidden from all players until he plays it. He may talk about it, but if he does, the OL can listen. The Spellcaster needs line of sight to his target to cast a Spell on it. When cast, a Spell is discarded and gone for the rest of the Encounter. If there are no Spells left in the Deck, no new Spell can be drawn. The discarded Spells are reshuffled in the deck of Spells at the end of each Encounter.
Starting equipment
Weapon: Alder Wand – ranged BY, ~ : +1 range, ~~ : If you have no Spell, draw a Spell
Trinket: Spellbook – exhaust: Draw a Spell. Spellbook does not refresh as normal; refresh Spellbook at the end of each quest.
Skills
Starting skill: Abracadabra . Draw a Spell at the beginning of each encounter. Add Heal, Protect, Teleport and Whiplash to your deck of Spells. Discard all your Spells at the end of each encounter. Pay 1 fatigue, exhaust: Cast a spell.
1XP: Inflame . Draw a Spell at the beginning of each encounter. Add Fireball to your deck of Spells. Abracadabra does not exhaust.
1XP: Exorcism . Draw a Spell at the beginning of each encounter. Add Exorcize to your deck of Spells. Abracadabra’s fatigue cost is reduced by one.
1XP: Mnemonics . Draw a Spell at the beginning of each encounter. Add Replay to your deck of Spells. Abracadabra may be played during other players’ turns.
2XP: Handiwork . Draw a spell whenever you draw a search card. Add Repair to your deck of Spells.
2XP: Warlord . Draw a Spell whenever you defeat a master monster. Add Obliterate to your deck of Spells.
2XP: Defeatist . Draw a Spell whenever a hero is knocked out. Add Revival to your deck of Spells.
3XP: Premonition . Draw a Spell at the beginning of each quest. You may keep one additional Spell between encounters and quests. Gain +2 health. Add Negate to your deck of Spells.
3XP: Pyrotechnics . Draw a Spell at the beginning of each quest. You may keep one additional Spell between encounters and quests. Add Firestorm to your deck of Spells.
12 Spell cards :
Heal : A non-knocked-out hero in your line of sight recovers 3 health.
Protect : A hero in your line of sight may reroll his defense dice. Whether he does or not, add 1 shield to the results.
Teleport : Move a hero in your line of sight 2 spaces.
Whiplash : Cast during your attack, after rolling dice. Your attack gains +2 damage.
Fireball : A monster in your line of sight suffers 3 health.
Exorcize : A hero in your line of sight discards a condition.
Replay : A hero in your line of sight may reroll an attribute test; subtract 1 from that new roll.
Repair : Flip a search card you carry face up.
Obliterate : A small minion monster in your line of sight is defeated.
Revival : Revive a hero in your line of sight. That hero recovers 1 additional fatigue.
Negate : Counter an Overlord card or a Plot card activation costing 1 or less Threat (the heroes still gain the Fortune); they have no effect.
Firestorm : Monsters in your line of sight collectively suffer 8 health in total, distributed as you wish.
Spellcaster is just an untested idea, but I would love to see something like that: a Mage class with a deck of spell cards.