Side View Space 'shoot-em-up' Mechanics

So, recently I have started writing a mall project surrounding a side view space shoot em up style game.

I was just wondering what kind of mechanics do you think would fit into such a game?

Can you think of any cool ways that the bullet damage could be calculated?

Do you have any other thoughts on that?

Let me know ^_^

Power-ups of course! A shoot 'em ups arn't fun with out power-ups. Have you played Gradius 1 or 2? Those are good games work off from.

I figured power-ups was a standard MUST HAVE within this kind of game.

I was thinking other things, like 'perks' or mechanics that work behind the scenes, during the 'shoot enemy and take damage' process - similar to how you make a battle formula, but for bullets instead of skills ^_^

Ah well in that case there's a wide range of abilities. There are shield effects, maybe you can have different size/positioned shields or have them able to damage enemies when the player rams them, then there can be status effects like slow down, reverse controls, and the ever classic poison.