- How many levels would you want?
- Keep the health bar or make it one hit one kill? (there will be a lives system either way)
- How long should the levels be?
- How hard should it be?
- Are the graphics alright?
Level-wise you should probably take a look at similar games (Tyrian comes to mind) and make a single, smaller campaign. A few ordinary levels, with one mini boss in the middle and one badass boss at the end.
I didn't even see the health bar when I was watching the video! So, you'll have to do something about that. Make it more noticeable. The unorthodox window size and a lot happening on the screen might have something to do with this as well.
How about putting the health indicator on the actual ship? Doesn't need to be a bar, but something that tells the player whether the ship is in spotless/good/bad/near-death state.
One hit, one kill seems like the game will be over too soon. Unless you're going for an unforgiving shooter experience, keep the health bar.
The level demonstrated on the video seemed appropriately long to me. I probably wouldn't go shorter, though. Unless you have a level containing only the boss, or some other special level.
Just right! You should make it challenging, but fun. As the game goes on, make it harder. Players will die and that's fine. Just make sure they won't get too frustrated with the game early on.
The graphics are quite alright actually. Did you make them yourself?
The big ships don't have to be so detailed. The player won't have time to admire the enemy ship in its entirety. Make the background or ships brighter. It might just be my monitor, but the bad contrast makes my eyes tired.
What bothers me is the amount of flashing projectiles floating in space. Maybe tone down the flashing a bit (lot)?
I think I saw some stuttering while playing? That would need to be fixed eventually. Although, I wonder whether you will be able to achieve that in GameMaker.
There are some commercial games out there that were made with GM, but you'll notice that all of them look and feel similar. GM is designed to be easy and abstract so you can make many different games. But this also means you can't really make the game EXACTLY as you want.
At least that was the case 10 years ago. I'm not sure how they fare today customization-wise, but I image learning a programming language will get you a lot further in the gamedev industry...
Or just hire some folk to make you game!