#1. When you die, you have 1 second to hit the bomb button 20 or so times (an invisible counter) and, if your fast enough, your ship will release one of your smartbombs, harming enemies in the proximity of your ship and recieving points for it. This would take away frustration from ALMOST getting that extended life or helping you damage a boss. Or pressing the bomb button one millisecond too late to save your life in the first place. Just keep mashin'.
#2. If the engine the game is created in supports it, say you reach a forest stage. Explosions on the ground and dust rising from craters tell you that your are being under attack from above your ship (this could actually be a very short movie sequence, say 2 seconds). Zoom in on the ship and watch it (and the background of course) revolve 180 degrees and start to fly upside down. You'll see the sky that is now underneath your ship, complete with clouds and a very screwed up perspective for a shmup, right? Also, some mountain tops could appear with enemies firing from "below". But perhaps this won't work, I haven't really thought this through from a design point of view.

#3. Cool (imo) concept for a smart bomb. When released, instead of a big explosion, rings of "hot air" expell from the center of you ship, and the background underneath will look like you're moving/holding a lense above it. Sort of rings on water. Ripple bomb?
I have some more game concept ideas, but sadly, they don't apply to shmups.
Emph