I realize that we no longer have a regular duffel around anymore and haven't missed it. Until we started packing for our next trip. We expect to return with significantly more items then we leave with. In the past we've done this by stuffing a fairly simple duffel type bag into our main suitcase. But I could use some recommendations.

-Sure, suitcases have expansions for this. But it is easy to get over a weight limit if you use it.
-The lightest/most packable option is a large stuff sack. Does double duty during the trip as laundry bag. We've done this, but the very minimalist set up makes it a real pain to carry, and I'd wish it would close with more than cinch strap.

Is there such a thing as a packable duffel? What do you use?

Ideally it is big enough when used to hold more than a carry on (although a carry on sized bag would have it's benefits too). And it would compact into a small and light space when not being used. It need not be super durable - by design it gets half or less use than your regular luggage.