Add More... Excitement?

A good friend played my current beta and suggested that I make the long stretches between items more memorable. I had planned previously to make them like Metroid & Metroid II- 2 main bosses and a final boss. This makes the areas between bosses like a pain, chore, and waste of time. I've decided to add minbosses/guardians to a good amount of the powerups.

Yakuza, the missile guardian. He doesn't utilize the missile's power, but he is weak to it.
It's hard to come up with how these bosses work. I want each one to be weak to or use the powerup they are set to guard, like in Fusion, which I can certainly say had some of the best boss battles in the series.

Nameless bomb guardian. Unlike Yakuza, he has absorbed and uses the bomb against you. His shell is too hard to be destroyed, you must find another way to damage him.
I'm hoping to make this game much more exciting with newer features, as before I had not come up with anything new, but rather, only re-hashes of previous Metroid elements.