Just a couple of ideas off the top of my head, they could possibly work stand-alone or even combined.
- Finale - As the shield ends, create some type of offensive capability (i.e. an Explosion) based on how many bullets were collected.
- Secondary Objective - While the shield is active, there are collectables (like coins in a platformer to help guide the player).
- Enemy Diversity - A Kamikaze-like enemy would greatly improve the difficulty/risk during shield mode.