That looks superb, I really wanna play it!
Some ideas:
* Make it possible to start further in the game, i.e. like if you've already played a while with more/harder enemies and better weapons. One of the things that made me stop playing Geometry Wars was how you had to play for some minutes every single time until it got interesting.
* Maybe a black hole like enemy like the one in Geometry Wars? I loved those the most. ^^
* Some kind of enemy that absorbs other enemies by touching, thus getting bigger. When you shoot it it breaks down into smaller parts.
* Maybe something like the Asteroids asteroids, just floating in the way (not chasing you) but you should shoot them so they don't get in the way
* Powerup: A point defense system which shoots only once every second (or something similar) and targets the nearest enemy (as long as it's close enough).
* Powerup: A small wingman ship which accompanies you and shoots enemies, maybe colorcoded, i.e. a green wingman will only target green enemies (just so it's not simply another weapon upgrade). May be useful for difficult to hit or other dangerous enemies
* Perhaps getting hit won't instantly kill you but instead you just loose an upgrade, thus the game gets harder when hit but you still have a chance to get back into the game?
* On the same note, how about you get more points if you use *worse* weapons or powerups, thus if you actively ignore them (if they are for pickup or if you can buy them or perhaps one button discards the last obtained powerup) you get a harder game but also gain more points. Also if gaining new powerups are tied to the score (either by buying or just every X points) this could be a way to quickly regain powerups after loosing them when you got hit by an enemy (see the point above) if you survive long enough.
* If you can buy upgrades use the score as the currency. This way if you get better guns and thus the game is easier you also have a lower score. Or you could try to survive a longer time with worse weapons but are able to get the better stuff sooner.
* Maybe use a Fantasy Zone or Cloud Kid kind of store, i.e. every once in a while an object or stronger enemy shows up and if you destroy it you can get into the store to upgrade some stuff.
* Powerup: Decoy. Single uses like a bomb; a decoy flies around for some seconds and all enemies (or maybe those nearer to it than to you) will target it instead of you.
* If there are different kinds of weapons (like spread shots, lasers, etc.) maybe make it possible that they aren't exclusive but fire all at the same time (like Heavy Weapon), thus giving extreme firepower. ^^ Would propably only make sense if getting hit only makes you loose one weapon instead of everything and if there are enough enemies to still make it dangerous.
Ok I'll shut up now...
