I agree that Guilds will utilize the 7 extra cards, but I think there will be only 6 guild cards (one for each player for up to a 6 player game). I think the seventh card will be a blank. And I also kind of think that they may be items that are gained by revealing cards in hand as an action. Players will be limited to 1 Guild per game. Thus:
Barons' Guild – Reveal 5 Estates to gain this card.
Nobles' Guild – Reveal 4 Duchies to gain this card.
Royals' Guild – Reveal 3 Provinces to gain this card.
Silversmiths' Guild – Reveal 7 Silvers to gain this card.
Goldsmiths' Guild – Reveal 5 Gold to gain this card.
Villain's Guild – Reveal 3 Curses to gain this card.
The cards themselves will be slightly unbalanced (like Prizes) but also more focused on what earned the player the Guild. Thus...
Reveal an Estate. If you do, gain a Gold.
Reveal a Duchy. If you do, gain a Silver and an Estate.
Reveal a Province. If you do, gain a Gold and a Duchy.
Once per turn as an action, trade a Copper for a Silver.
Once per turn as an action, trade a Silver for a Gold
At game end, all Curses are worth 1 Victory Point.