My guess is there will be five non-kingdom cards - these will be the Guilds.
One possibility is that they'll be permanent rule-changing cards, like "Whenever you gain a Victory card, gain a Curse" or something like that.
Another is that you choose a Guild at the beginning of the game (or choose through a kingdom card), and it gives you a permanent bonus, like "At the start of each turn, +1 Card" or something like that. You would only be allowed one of these. If this were the case, there might not be any new kingdom Victory cards, to allow for a 6-player game. Or it could be five, and the 6th player would just be screwed.