It's already been said, but a place to clip a blinky on the back would be great (I have a taillight on the seatpost and one on one of my panniers, but I'd like to have one on the helmet as well).

Also already said, but more reflective material: I bought a few rolls of the 3M tape from a place I found out about here on BF (that I just saw is going out of business - dang!). I put black tape on the styrofoam material along the back and blue and white wherever I could fit pieces. You can hardly see the tape most of the time, but it gives quite a bit more visibility when light hits it.

Finally, a big plus one on speakers! I will not wear earbuds on the bike (that just seems crazy to me!), but speakers would let you hear outside sound while still enjoying a little music.

Just my cent-and-a-quarter (I think you have to be a member for 5 years before you can give a full two cents)...