Either for use by any Mage as an alternative to "normal" casting, or perhaps for a new Arcane school mage special ability (though I strongly advise the latter):
Counterspell would be an ability, rather than a spell. The ability would allow the possibility of cancelling a spell currently being cast by the opposing Mage. You would have to have the same spell available to cast (if it were your action), and then cast the spell (during the counterspell step) using your quickcast marker, and pay the same amount of mana that your opponent is using to cast his spell. If this is done, then the original spell fizzles.
So, if the enemy mage spends 12 mana to try to teleport your mage to a zone on the other side of the board where a bunch of creatures are waiting for him, you could reveal a Teleport spell in hand (or use one from a Mage wand), flip your quickcast marker, and spend 12 mana yourself to cancel the action.
If you want a little more flexibility at the expense of complication, you could alternately allow the use of a spell of the same type from the same school with a higher base spellbook cost instead. So, for example, to cancel a Minor Heal, you could use a Heal if you had no Minor Heal in hand... but you only would pay the cost for the Minor Heal.
For spells with more than one school listed, you could use a spell from either school... but the total base spellbook cost would have to be exceeded. For example, you might be able to cancel a Renewing Rain with a Lay Hands, but not with a Heal.
I would think that this could only be used on Attack and Incantation spells for flavor (if not logistical) reasons, and that you would have to be within range (using the range of the spell you are using to counter the original spell) of either the opposing mage, or the original spell's target in order to be able to do this.
I am suggesting that this would be an ability specific to a new mage, rather than for use by all. This seems as if it would particularly empower melee mages with added magical defenses... they could prepare teleports every round knowing that this can help them defend against being teleported away, all the while using melee attacks instead of (or in conjunction with) the teleport counterspell.
It seemed interesting... wondering what you all think about the concept.