This is what happens when Retro Blazer, Chex Quest and Adventures of the Square had a baby. I liked the gameplay, the variety of weapons/monsters and the awesome time freeze powerup. For a demo level it was designed really well. I know that this level does not reflective the quality of the final game and level design but here are 3 suggestions:
1. Don't give the player all the weapons in the first area of the first map. Make them earn their weapons/powerups as they play through the entire game. This makes it more of a challenge and gives the player more reason to explore their environment.
2. The maps need more verticality; they feel too flat. I would like to see, deathtraps, steps and 3D floors for catwalks and walkways. This is designed using the GZDoom engine, so you might as well use its features. 3D floors and fake water are not that hard to set up in GZ/Doom Builder. Old versions of ZDoom even support them.
3. I don't know if the game intentions are to be more retro or modern retro. If it's the former, you can draw a lot of inspiration from Heretic with its awesome set of powerups. If it's the latter, you can grant the player free movement by allowing jumping puzzles and platforming segments. This would give the maps more variety and take secret finding to a whole new level.