How about powerups? like, there could be a random factor that would activate each level, and the powerup would be used for the next lv (ending at the end of it). The list of powerup could be:
2x bonus - all gold given by doyus multiplies by 2.
Faster - All defenders speed increased.
Nothing - No bonus at all.
Free build - Ability to build a especific defender a few times for free.
Help me finding more cool powerups / tell me what do you think.