It would be interesting to introduce a mechanic where flames near water could "dry out" the water after some time. Players couldn't hide in the water forever.
The lengthy arrow combo training challenges are very frustrating. The targets appear and disappear almost faster than the game animation will allow you to shoot. :/
I would love to see the addition of "special abilities" that can be activated by a (like space bar). Each new family member could have a special ability, and the player would have to choose which ability to have available. Special abilities could be ranked up, too.
e.g., Grandma could race across the screen in that wheelchair and destroy bad guys like a lawn mower. Mother could call in an air strike.