Great to see some enthusiasm for this
I think the basic commands would be:
circle row col radius #RRGGBBAA fill [linewidth]
line row1 col1 row2 col2 #RRGGBBAA
tint row col #RRGGBBAA [linewidth]
Circle: Draws a circle of the specified radius with its centre at row, col. The row, col, and radius could be floating point values so that you can specify position and sizes on a sub-square resolution. The colour could be separate RGBA values but probably perhaps a #RRGGBBAA hex representation is the simplest. Fill would be 0 or 1 to specify a circle outline (0) or solid circle (1). If fill is 0 then linewidth specifies the thickness of the outline to be drawn - although I'm not sure if this should be in screen units or game units (i.e. 1.0 would be as thick as one game square) - logically it should be in game units, but as a visualiser aid it might be more practical to be in screen units so it looks the same on different map sizes.
Line: Draws a line from row1, col1 to row2, col2 - again these are floating point values so that you can draw lines from or along the edges of game squares. The integer at value would be the centre of the square, so for example the very top left of the board would be (-0.5, -0.5).
Tint: This would overlay the specified (integer) game square with the specified colour (users should use a transparent colour unless they want it to completely obscure the game board).
How's that for a starter, any ideas/comments/improvements?