I like this little game. It’s essentially a 1:1 recreation of something I played as a child. Players enjoy character progression, and this game delivers that well. A great design exercise is taking an existing game and modifying it to improve it or make it more interesting. In your version, the expansions include three upgradeable skills and a final boss—whom you can even talk to! :)
That said, I do have a few reservations. I didn’t really feel a sense of progression regarding the skills, and as for the boss—it’s a shame that players can’t immediately retry the fight after losing. In game design, we need to apply heuristics! You should ask yourself questions like: "Won't forcing the player to start over after the boss fight just frustrate them?" "Will the player actually feel the 'necromancy' theme if 80% of the gameplay involves eating fish?" or "Does the player truly feel a sense of progress when they spend points on a skill?" It’s cool that the boss is there; I appreciate the humor.