If I were to spitball ideas:
Keep the Brick, Roof, and Tower the Same
Have the Brickwall Gain 2 For Each Adjacent Structure, Count as 2 Structures, Give Defense Twice, or some combination of these.
Have The Balloon grant gain no additional defense for adjacent structures, but grant 1 Haste for each adjacent structure. This is a good payoff for it being hard to combo with the other structure pieces.