I am thinking of having the last level of each chapter be a bit more like a boss fight, in the last one of the demo has one but while it kind of works as a different kind of fight I don't feel like it succeeded as an actual bossfight.
When I finish the game I'm thinking of 9,99$. The remaining content would be two chapters that introduce more new stuff (new look for each, new enemies and weapons) and the epilogue (remixes of previous maps so that the player can experience them differently with all the weapons and enemy types).
Plus I may actually add a small story to it, a cutscene that plays at the start and end of each chapter to give a bit of context and an overall progression to the game. Wasn't planning on that but kind of figured it out that it might not be hard to implement and it will be fun to do while adding something that was lacking in the game for some players.