I do love a mech game. That said, this feels very early in development, so I'll critique what is here rather than what's missing.
The tutorial is good in the sense that it focuses on the right things. You aren't going to move at all unless you press wasd, so that's going to be obvious, but then you have a sign to press shift to sprint, which the player might not know about.
The mouse movement feels a bit rough. I don't know exactly what's going on with it, but I think it's a symptom of not using raw system mouse input. This was especially noticeable when using the scope on the rail gun. I also found the middle mouse as the scope button tricky because I usually scroll with my index finger and so I couldn't hold it to shoot intuitively. I'd suggest a setting to toggle scope on button press vs holding it (I didn't check so you might already have this.
I have no idea what fortress mode does, but it sounds intense. I also enjoyed the secret room.