If it's not something they have published before, you have to explain to them why this is a hot new idea that's going to make them tons of money. Determining your market will have determined quite a bit of what the concept is. Length, degree of explicitness, orientation, flavor, literariness, etc. Now you refine it. Again, what terms you get for the book and what terms you can offer the writers will depend upon the publisher.
Good outlines get the right kind of fiction on time with a minimum of paperwork hassle.
Your workshop may be different. Send copies of guidelines to your agent. Juggle submissions for length, style, subject, tone, ect until you have enough words to fullfill your contract while reinforcing and highlighting one another. The sum of the parts should be greater than the whole. Think deeply about the impact your anthology is going to have on the reader, and tailor acceptances and rewrites to enhance that.