Hmmm areas to improve... I can think of things it could be smart to avoid or to strive for, based on what's done right here.
- Avoid scaling health to increase difficulty on enemies, and keep player health scaling small. Make a 5% increase to health feel huge and have a real impact on your strategies. A unique way of doling out rewards in strength can be through your actions (think Elder Scrolls series) and fits with an evolving organism. Your physiology (traits and appearance) changes on a curve instead of in steps, if that makes sense, based on how you play. Make you grow your own character, literally train it, instead of picking up upgrades or selecting from a menu.
- Make retries and/or replays the focus. Try finding the rhythm that prevents the player from relaxing their controllers, but doesn't feel rushed, for a death/retry loop so they can stay in the flow. If your play style changes your character over time, it's tempting to see the results from playing differently, and players gets to see more aspects of the game.
- Don't do procedural generation. It's OK to replay the same areas on multiple playthroughs if the combat is varied enough, and it often can just make your game less interesting as players instinctively notice no matter how good it is.
- The concept does sound good for a roguelite, so it's tempting to go that way, but my instinct tells me it will work better as a defined progressive story you play through 1-5 times depending on the player. Better to be short and sweet though, than long and tedious for this concept.
- Don't indulge too much in particle effects or light sources, less is often more and shouldn't be added early cause that constant slowdown in launch time and framerate really adds up to the frustration and lack of momentum when developing. There's other ways of making it feel good early on that doesn't affect performance, like slight camera shake and physical feedback. Write yourself a nice and versatile camera shake function and you can use it for both combat and story telling, even in cutscenes. Your cannons in this game felt really weak compared to their actual effectiveness.
- If you're lucky you can get someone dedicated to sound. Sound design is going to do a lot for the feeling of being inside a body, like a muted no-reverb kind of soundscape with "squelching" and indistinct bodily sounds, and if you do that right it's going to be the X factor that makes the environments memorable.