AMD's style seems to be incremental upgrades, so I wouldn't expect to see anything more notable than a small bump in performance possibly coupled with a small decrease in price. In other words, buy now if you need it now.
Look at it like this, if you had needed an RX 480 but waited for the RX 580 to come out before you bought a card, what did you gain in a practical sense? Nothing. Meanwhile you lost all that time you could have benefited from the 480.