Update!
Been getting a consistent amount of work in for a few months now, 10-15 hours a week. Made progress in most areas: levels, gameplay, etc. Even with that much time going into my game each week, I'll need to narrow my scope if I want this finished within the next six months. I wanted to do bosses, and maybe some non-dialog cut-scenes, but I think that's a bit of a stretch at the moment. I'm putting my efforts towards the gameplay and level design, trying to find the right balance of puzzle, action, and exploration, while at the same time offering something new and different. That's the goal. If everything falls in place nicely, then I might do the extra stuff. But if the game feels complete, then I'll consider it done.
I've been experimenting, trying to discover what I can do with the elements and mechanics I already built in, instead of adding more. It's a good way to not expand scope, make the most of your level design, and discover new ideas that add originality. I think I'm getting somewhere. One example is that I found the open areas to be more fun than closed, narrow passages, and that the trees add an excellent strategy element; you can blast out a path of trees to take cover, or as an alternate path around tense areas.
When I started this project, the level design leaned toward the classic "find key here, back-track to open this door, to hit this switch that opens another door" pattern. I've done that before, so it felt like retreading old ground; and second, everyone's played that type of game. I want this to be something fresh. So the more I've worked on the game, the farther away I moved from that. It still has those elements, but I try to use them sparingly.
You might notice that I removed the fuel bar. I decided that having to watch your ammo and collect fuel pickups was too distracting for this type of game. And if you are new to a level, you don't really know how generous it is with pickups, so you might conserve your ammo instead of blasting away trees like you're supposed to. So instead, I gave the player infinite ammo, since you rely on it for almost everything. It feels more rewarding.