RE: adding features
I think it's a good mindset to try to keep the game simple, but I do think you could expand it without damaging the game. As is, there aren't a lot of choices to make. In a game where it's really difficult to play perfect this is fine because where your attention goes is how you create your play-style, but in this game, I've played for about two hours and I think I'm closing in on that perfect game. If you were to add features, I think you should try to make the game more complex from a strategy perspective than adding more to manage. That said, you made a better game than I can so I trust your judgement
RE: Levels vs Roguelike?
I think it depends on what features you add. If you want to play with the nature of the game, I think levels is the way to go. You could have one level where you are told in advance which color bombers are coming and gain the ability to change your arming slots so you can produce enough of the right fighters or one where it's cloudy and you can't see some of your slots so you have to remember what you put where, one where almost all your slots are full so it's hard to move planes around, etc. I think that would take a lot of work to make cool. It would probably be easier to make it rogue-like so each bomber destroyed gives you xp or money or something you use to upgrade/buy planes and the fun of the game comes from experimenting with upgrades to figure out what deals best with the more difficult waves. I think the levels sounds cooler but easier to mess up and more work