If I could promote a 'feature request' for AFNI, it would also be to allow two overlays to be selected dynamically in the main controller window. My reasons aren't purely functional but DTI-tractographic. It would be really nice to have the ability to overlay both a network of target ROIs and the tractographic results associated with them.
I see there are ways of doing this offline (-> with 3dcalc), however when making images it is often quite useful to be able to dynamically interact with the color, thresholding, brightness, transparency, etc.
Having such functionality would be a boon when combining AFNI and SUMA as well, since the latter has such increasingly nice target ROI+tracking tools...
--pt