Will the other world championship skins be available when the new SKT skins come out?

More than likely. That's usually how it is. When the new ones come out they usually bring back the past skin of the skin line. But its probably only going to be in Bundle form.

Ya sadly that is how they been doing it lately. I mean its kind of good if you are someone who wants to "own them all" but if you only wanted one then ya you really are out of luck. If anything you can try your luck with Mystery Gift, or Hextech.