one thing i whould like to see on a complete version whould be an infinite mode with randomly generated levels (it could be a button on the dialogue that ask if you want to continue and instead of playing again you whould continue to defeat the rest of the mosters and this whould continue infinitely)
for this infinite mode the shop could upgrade after each 2 bosses - the 1-sword could desappear and there whould be a 5-sword (same for shield), the heal could get a 3-heal and the gems could get directly a 4-gem.
the enemies could get harder and harder overtime (if you think why whould there be stronger enemies if i alr saved the princess, it can be that an enemy was getting the pricess to its master)