I would also say multi colored enemies. Now would it be possible to utilize the ball or missiles when not in use to provide highlights to the player?
The kernel options I'm using for the standard kernel kill normal use of the missiles, but they can be brought back and used in a limited way:
(I meant to add that info to the bB page years ago. I just added that section last night, so you might need to refresh the page.)
The ball is only visible on rows that aren't black, so its use is also limited.
Just another random suggestion; Maybe for one level, instead of having the cool Laser, you make your ship have a single color beam, and then have both multicolor enemies and multicolor player ship...Maybe here the enemy ships require 2 hits or something...Either that or maybe you change the color palette in some way...
I was always kind of a fan of the color palette changing to show progress...
That thin line that the ship is shooting can be any color. Maybe the player could upgrade his or her weapon and the color could graduate from blue to green to yellow, then to red.
Instead of forcing the player to fight through a set pattern of bad guys, then fighting a "boss," maybe the player could choose when to fight the boss. Maybe the player could fight enemies of a lower rank until the weapon is upgraded to red, then the player could go after the boss since that would give the player better odds. It might be possible to defeat the boss with a weapon that hasn't been upgraded to red, but it would be much harder.
None of this is written in stone. I'm just shooting balloons, bubbles, rainbow hard candy, and glitter out of my ass and seeing what sticks to the wall.