I's like to see support for JNG (JPEG Network Graphics, basically PNG with JPEG compression, or JPEG with alpha-channels).It doesn't necessarily need to be soon, but I'd like to see it.
My reasoning is because I've been keeping up with Mozilla foundation's Bugzilla about re-adding MNG support to Firefox, and at the moment it looks like it may happen via plugin soon, and as JNG is part of the MNG format, it'll be there too, which would be very nice filesize-wise, having an 88.5KB file to download on a page over the 628.7KB equivalent PNG (Example here)