Suggestion of tiny scale, related to one of currently existing mission types: Key Rush.
If you are unaware, the mode is about nothing more than a huge boost to key drop frequency along with only single playability of the mission, leaving it to the player as kind gift to put your hands onto.
Precisely, key opportunity on enemies to appear is heavily multiplied, plus you get a few extra keys each time wave has been cleared.
What I thought could give it a nice extra touch is that whenever a wave comes to play with only enemies that cannot be targetted for dropping keys (such as Asteroids), game has premade variants for those waves on Key Rush, and those will get a bunch of enemies outstanding from the rest with key drop aura.
For example: on Asteroids through the wave duration one or two asteroids will fly through on much lower speed, giving attention to the player and time to be destroyed, dropping keys for exchange of doing so.
All of this has absolutely no significant impact on the gameplay, but rather being just a pleasant aesthetic addition to this mode, giving the effect of even more diversity and effort put into the game overall.