I really really like iNano's idea.
It feels like a really subtle ability which can mess up the opponents game in ways that snowball into a victory.
Afraid to see it in the hands of the likes of Charmyna or Keechjen though
I would maybe even allow the ability to be used before your own action phase, so you can force your opponent to act twice in a row. There might be very few situations where one would actually want to do that, but it gives more versatility without seeming to strong.
Another Idea in the chronomancer direction: (I am stating it in its bluntest form and then it can be trimmed from there)
When casting a spell or revealing an enchantment, this Mage may decide not to pay any mana. If he does so he receives some marker. During upkeep he has to pay the mana + level of the spell. Obvious variations would be to only allow reduction by the level and require to add a fixed amount of additional costs.
Also: What happens if he cannot pay?
Lose the game?
Take as much damage as mana is missing and opponent gets that much mana?
All the mana gained is immediately used to pay off the debt and so he cannot cast anything until the debt is paid off?