Congratulations, lots of good things going on in this game.
You probably know better than anyone what features you could implement and still be worth while regarding to your intended price point. If you decide you want to do more work on it I would suggest the following:
Drag the tiles across the screen instead of the system you have, not sure why you have the clicking on blank squares part, you should think about removing that.
Since you would be dragging the squares around you would not be as reliant on the bottom picture as you could easily move them around until they started to resemble something. Then you could slowly reveal the bottom over time, as suggested above.
More levels, surly adding more levels would not be hard since you have done all the hard work, 15 levels at a minimum I would think.
Like I said, you know what your budget and price point is etc, so you are best placed to know whether changes at this point would be beneficial.