I think one feature that would be really useful would be to allow the user to enter a puzzle into a blank grid and then allow them to solve it after first checking that there are no errors and that it's a valid sudoku. If the validating routine can also grade the sudoku and assign a difficulty level like Easy, Medium, Hard, or Extreme, that would be even better.
This is the best Soduku I have used. There is only one feature I have seen on other Soduku apps which I don't see in this one. I like the ability to store bookmarks of a puzzle at various points during my solution. this replaces the need to undo and allows me to be more intentional about when I will need the ability to undo pen and pencil marks.