was actually a bit of info on a single slot 480 by HiS coming soon a while back. not seen it released yet but it was shown off at one of the tech shows.
was still long but a true single slot card. but i have not seen a shorter "mini" card advertised yet. plenty from the new nvidia cards all the way up to the 1080 but none yet for the rx 480 cards that i have seen.
it should be possible since they have done it with the 1070/80 cards which use more power than a reference 480