A slight pause in the animation sequence (the frame before shooting) depending on different firing mode (e.g. No pause for snap shot, 0.5 sec pause for normal shot and burst, 1 sec pause for aimed shot, etc., something like that, so not to slow the game down too much) would be a nice touch. It would give a visual difference between the modes and make each mode feel a little more unique.
For aliens (I assume they use different firing modes, or will) you would be able to know what kind of shot it is attempting, and add a little more tension.
Current: Crap, he's shooting at me, I hope he misses.
Proposed: Crap, he's going for a norma... wait, an aimed shot, I'm screwed.