first here is a new mechanic that could be interesting:
Defector. Cards with the defector ability are pretty powerful but the drawback is that they "defect" to another player under some conditions, though they lose some of their power upon defecting. I think it has potential for interesting cards and abilities. Here is an example:
Sacred Pond
Land (Rare)
Defector: when the owner of Sacred Pond is dealt damage by a source an opponent controls, that opponent gains control of Sacred Pond.
T: Add 1 to your mana pool.
T: Add 1 mana of any color to your mana pool. Use this ability only if you are the owner of Sacred Pond
Flavor text: The pond dried out as the invaders drew near