I love the Droid as is, and have zero regrets. However, if I were to be offered such an honor, I would:
Make the slider keyboard a dedicated touch screen keyboard.
Make both the touch screen and the touch keyboard sensitivity user-adjustable.
Make the touch screen stylus capable.
Improve the microphone.
Make the speaker grill on the back thicker to increase durability.
Replace the stock music player with one that includes music, photo & video files.
Reconfigure the 3.5mm jack so that a microphone could be inserted for use in lecture recording type apps.
Incorporate a tazer into the top with low profile electrodes. (-Purely a "just in case" type of thing.)