I think one thing that would add more depth, and also strengthen the theme, would be to add other chess pieces to the player that are only able to making non-attacking moves each turn along with your king. The new goal would be to protect all your other pieces, which could start as a an extra piece or two, and expand to more pieces. This would add a lot more depth and variation in challenges you could provide. You could even have two kings to protect all the piece you have, and now you have 2 units to divide your movement cards between. Just some ideas.