My ideal test card would have a combination of a grid , colors, letters & numbers and shapes.
Grid: a line every 20, 40 or 50 pixel is great but it would be great to have the option to use the bottom right or the center as a start point. Also having the center of the grid (XY coordinates) with bolder lines when you use the center of the image as a reference point would be great.
Color: the eight color band inside the 20/40/50 grid in multiple location (depending on size) but one needs to be in the center of the image.
Letters & Numbers: Size of test pattern is a must and other letters numbers to be used to focus
Shapes: having at least one bold circle is important to get better geometry with extreme maps. The radius of the largest circle should be the smallest size of the test card rounded down to the 20/40/50 increment. Here is the formula I would use in excel to find the radius: =(ROUNDDOWN(MIN(Width:Height)/Grid_Size,0)*Grid_Size)/2
I'm sure there are other things but this is what I can think of now and hopefully it all makes sense!
Thanks for jumping on the band wagon