Full proposal or not?

I'm working on the concept for my next book. I've got a two-book contract with my publisher, and the first of the two is now in design, so it's definitely time to get on with the second one. The topic was left open when the contract was signed. I sent a list of ideas to my editor, she discussed them with her managing editor and the president of the company, and they came back with another idea I hadn't inlcuded. It was actually a topic I had considered but rejected because I assumed other books already filled this niche. I reconsidered, decided it wasn't such a bad idea, looked for the competition and found none, so now I'm working on an outline.

So, here's my question:
Do you think I should do a full proposal, including a discussion of the market and the books already available? Or can I assume that since the publisher suggested it, all I really need to address is the content? This will be my third book with this publisher, so I don't need introduce myself or establish a platform. I'm planning to submit a detailed outline and discuss how I'll approach the topic. I don't want to undersell the project, but I also don't want to waste time doing unnecessary work. Is there something I should include that I'm not thinking of?

Well, yes, that's obvious, but right at the moment I'm trying not to bother my editor at all, because she's involved getting this big complex project (my just-completed book) into production, among other things. I want to submit something to her for discussion before I demand a response from her.