I completed my first novel in what I expect to be a trilogy. Weighing in the difficult battle it would take to get an agent to agree to push three books (from an unknown author no less), I was reminded of something Terry Brooks had done.
He said when he was writing the second book of a trilogy, he sheltered it from his agent with the hope of surprising him/her. However, when he was finished it was too full of holes to fix, and he had to start from scratch.
If I do get The Call, and they ask what I've been working on, I'm going to be at a loss. Terry's experience made me cautious about starting the second book in the trilogy, but I want to write it. Now, I could write a stand alone book and try to snag an agent and market the trilogy that way, but if they didn't want the other books, it would be a sticky situation.
What should I do?
Publishers like series from first time authors.
Write the second book.
I have NO idea why Terry Brooks had a book full of plot holes, but I assure you it's not the norm. I have authors working on series ahead of editors and they've never had to trash the whole book and start again.
Generally an editor isn't going to overhaul a book so completely that a sequel would have to be started from scratch. Tinkered with sure, but that's about it.
And remember too: what you want to write is the most important thing right now. Don't write trying to anticipate what an agent or editor wants. We don't know what we want till we read it, and only you can write that.