Thanks for the giveaway!
I would like to see the Equipment mechanic from Magic the Gathering: cards that aren't monsters, but are attached to a monster to give it an advantage.
For example, there could be an Equipment that would give +X Health to your frontline monster. When the 1st monster is killed, the equipment would also be destroyed.
This would allow a player play MORE than 7 cards max (1 summoner + 6 monsters), as you could spend you mana to play extra Equipments.