A Sudoku gameboard consists of 9 blocks in a 3x3 matrix.
Each of those blocks consist of a 9 tiles in a 3x3 matrix.
To continue the pattern, we'll also represent the 9 placeholders by a 3x3 matrix similar to that used in dice: one dot represents a '1', two dots a '2', etc.
As a side effect,
all the patterns can be somethingmorphic, ie: look the same viewed from either side of the board.
To complement the dot notation we can use a simple variety of gameboards to add another element to the game...
Easy: the gameboard is divided into its blocks by thick lines, then squares by thin lines. The patterns used to represent each of the numbers 1-9 are consistent. (ie: this is a normal board)
Medium: the gameboard is divided into a 9x9 matrix of squares by thin lines (this is not much different from 'easy'). The 3x3 patterns are consistent.
Hard: the gameboard has no dividing lines and is a 27x27 matrix of dots. The patterns are still consistent
Evil: no dividing lines, and there is no consistency to the patterns that identify the 9 placeholder figures.
That last one would look like QR-code or braille, and could be made such that both the beginning puzzle and the end map draw a picture made of dots.