I don't know if the following was already asked or sounds bad but I got some interesting suggestions:
1. Add customize background instead of only black.
2. Lore of the following bosses like 9 or 10 for total peak. (Having a peak theme is good, but having a lore itself makes it genuine).
3. A boss where you should stop opening tiles when active and if you somehow open twice, all bomb explode and lose the game.
4. Add a diffused boss sprite in title screen where it meant you finished the following chapters which act as a trophy or achievement