Sounds pretty boring to me. I don't like the shallow story, the rigid lines in gameplay. I really like story-driven games, like Iji, a FREE indie game made from Game Maker. You had to mix and match your weapons to get new items, you could unlock many, MANY secrets, and you could change the game's story. Those are the type of features I played the game for.
My suggestion is to make it a bit more story-driven, like Iji. Download the game to see what I mean.
It's a bit boring to have just a shooter game. It'd be nice if you have an inventory, special items that are secrets or gained from bosses to create more weapons, etc. For example, instead of just obtaining new weapons, you can get modules. For example, you have the Shotgun and Blaster template. Then, you start off with the "Normal" bullet template. Then, after you defeat the "Fire Robot" boss, you get the "Fire" bullet module, which you can create a new shotgun type or a new blaster type that has the "burn" status effect. Plus, you can re-use the graphics by just re-coloring the bullet and making a few more animations. This would increase the weapon system's depth.
It would also be interesting if you could add in "data" logs or something. Create a story. For example, while going through the stages, you could find "data" logs that you can read. Get data about the robots, about daily life, about pondering on the philosophical side of AI. For example, do robots actually feel emotions, or is it all simulated?
This is why Iji was excellent--the overall story and gameplay never really changed, but you could avoid boss battles, get achievements, and unlock secret levels. This is why it was fun for me.