I really like the current stickers with the simple logo, but of course it would be neat to see a greater variety of stickers or what else someone might come up with.
Maybe a sticker design contest could be fun?
It might also be nice to see some proper bumper stickers.
I usually find bumper stickers of any subject tasteless but those who would enjoy one it would help to increase Ubuntu's and Linux's public profile while potentially generating money for development instead of spending money on advertising.