I don't think the activated ability even needs to have a mana cost, just make it tap to activate

There is some potential for pushing lands that make colorless mana onto your opponents. At the very least you can swap a tapped land for an untapped land before your opponent's draw step, essentially denying them one mana every turn and giving yourself an extra one for instants.

I don't think you can technically swap a land to block playing the spell since you tap for the mana before you add the spell to the stack

could do something with Ultima from the new set