Personally, given just how a 150 card expansion would have to work, this is my thought:
130 kingdom cards + 13 randomizers leaves room for 7 more cards. These 7 will be the Guilds referenced in the title. Each kingdom card will reference Guilds in a vague way, so it will apply to any of them. Whenever a Guilds kingdom card is out, the stack of Guild cards will be put out, like the Prizes. They'll have a higher cost (let's say $7), and when you reach $7, you can purchase a Guild - only one per player. Depending on which Guild you choose, it will provide a bonus to every Guilds kingdom card in your deck. For example, it might make every card in your deck part Victory (making Scout even more overpowered!).
That one should be called the "Toastmaster's guild." You know, because they celebrate everything!
I really like your general idea though. Contrary to eHalycon, I really like the idea that the Guilds could apply to all kingdom cards, because they become more significant (though even harder to balance). I'm not sure whether they even need to be purchasable -- they could just exist outside the supply and have the stated effect on the entire game. Like, whenever you use a card from the Guild expansion, you put out a randomly selected Guild card, which has its effect on the whole kingdom for the whole game. That design has the benefit that all players still see an even field. If you could buy the toastmasters guild for yourself, and your opponent can't have it, then scout and crossroads become insane for you, but normal for your opponent.
If the always-out design is used, the guild should be an optional effect that requires some cost to activate. For example, if the Toastmaster's guild is in play, then at the beginning of each turn, you may discard a copper to receive the toastmaster's guild effect for the rest of your turn.
Edit: I really like this idea. If Guilds doesn't do this, I might try to do a fan expansion implementing this mechanic.