Well, an asteroid field is a classic terrain type to use. That will keep the player on their toes. Mix in armed and armored asteroids that shoot at the player.
You can use a slow moving comet with a big particle tail that spawns enemy units.
You could have a nebula wave where enemy units fade in and attack.
You can do a ringed planet fly by where the screen is full of ice shards that fracture into smaller shards.
You can add a singularity or other hypermass and have a level where all the shots are warped in their flight path.
I hope those help.
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TAZ
