A "feature" I would like to see is that all PNG's would be (losslessly) optimized using PNGOUT (http://en.wikipedia.org/wiki/PNGOUT).
For example the Simple Machines logo would go from 3.046 to 1.524 bytes. I know it only saves 1.5kb, but boards usually tend to have a lot of pageviews, so any savings in bandwidth would be welcome.
I know not all gif's would become smaller when converted to PNG because of the small resolution (even with this tool) but some are. With the many advantages of PNG (like the option of 24-bit images, which could be handy for skinners) it could even be advantageous to completely switch to PNG.
update: I've tried some images from this board, and many of the very low resolution GIF's become larger when saving them as PNG. For example: the e-mail button goes from 135 bytes to 190. It's just 55 bytes extra, but on the other hand: it is 40%. I'll just stick my suggestion to optimizing the images which are already PNG.