It is a really neatly done game the Gameplay loop is really nice. Art is simplistic yet nice.
A varying environment would be cool, game could use some powerups to make the progressions feel nicer and have a difficulty curve as well.
You could have it endless or have levels where you break out of the cage, And You can add some bosses in it as well.
Really nice game, Played it for quite a bit and it could be addictive in a way.