I think your best bet for a romance people would want to read is all single people that end up together. If you take an already devoted couple, it's going to take a very well written novel to convince mono readers that looking for a 3rd, 4th, etc. is a romantic thing to do. If you take 3(or more) people that are thrown together it's much easier to romanticize without having to explain why your main character is in a great relationship but wants more.
There are a lot of healthy dynamics that would just take far too much explaining to work in a book you want mono readers to enjoy. If you find something that really works for you and gets published, you can always have the same people in book 2 and introduce more people.
There are some books already out there that start with a mono situation and then gradually become more poly. I remember the Anita Blake books did that, although I found them unbearable to read once they became more porn than anything else.