Suppose I was working on a mobile game with 10 missions, where each mission consisted of several levels. Would it be bad to release the game with only five of the missions completed, then add in the rest later through updates? The missions that are released would be completely polished, of course. I think it could be good because it would keep players coming back to play the new missions as they are released. Also, this game will be free, so they won't be paying for something that is incomplete.

AFAIK updating mobile packages is pretty troublesome. It would be a newer version of the game and would probably require a fresh install rather than just adding some files. Or perhaps an installer that checks for earlier installs and adds files to certain location where they can be picked by library loader of your game. You'd have to find out about your specific platform about the technical side, just make sure you can do it the way you intend to.

But I think you should rethink the way you market your (free) product. Why not market it as a game with 5 missions instead of making it sound shady with 5 plus something you'll maybe add later in some form? People will be happy with 5 missions and the 5 you plan to add later have little significance before you actually have them. The promise about the expansion can mostly do you harm if you can't quite finish it for some reason in given time or at all. Sometimes people are furious over that kind of things even if they paid nothing.

Be honest about what you have when you have it and make no promises you aren't sure you can keep.

Yeah, I was worried about mobile package update causing issues. I'm excited about putting my game out on the market, but i guess I'l have to go a bit longer to finish all the levels.