Gabe,
I purchased your app (great price), and here are some ideas:
1) Allow for different color chalk - yellow, orange (for Halloween), green and red (for Christmas) - make sure that the colors look like authentic chalk - the white that you are using now seems a bit too white and not chalky _ I' not sure if you know what I mean;
2) Allow different colors to be used on the board - so I can choose to use white, and then I change to yellow and write with the yellow (but what I wrote in white remains on the board);
3) Different board backgrounds - green, brown - I have seen those two colors besides black - maybe others exist as well;
4) Allow for different thickness of chalk - so that lines in different thickness may be drawn;
5) Maybe allow for different borders around the chalkboard (trim that goes around the four sides to customize like a photo;
6) Allow what you wrote/designed to be saved to Camera Roll and/or e-mailed;
7) Sounds will be great - including a normal chalk sound, a screeching chalk sound, and a fingernails sound;
8) An eraser would be great it we want to erase only certain parts of the chalkboard (I know that you can erase the whole board by shaking the phone);
9) The ability to draw a dot - when I tap, it does not draw a dot - so I cannot draw an eyeball as a dot, for example - I'm not sure if that can be fixed.
Other ideas (this takes you away from the chalkboard theme, but it will make it more useful to a wider audience):
1) Cork background - I know that it wouldn't be a chalkboard then - I just like cork - haha;
2) A whiteboard/tanboard option, with different color markers instead of chalk;
3) The option to put vertical lines and/or vertical and horizonatal lines (similar to a notebook page) - for people who are learning to draw or who want to take notes;
4) The option to use a picture stored on the phone as the backboard so that people can draw on their own pictures.
Cheers!