I wouldn't mind having just one folder to save rmail in. I've got this with obsession with wanting my inbox always clean, pretty and empty. But when I get a few rmails that I want/need to keep, they sit there at the top of my inbox bothering my mind. I know most people don't mind that, but...well, for whatever reason I do.
A seperate folder to stick these few rmails would be nice. But its size should be smaller than the inbox, say...10, 25, or 50 rmails. And maybe you have to pay to get this folder? Or even buy it in different sizes? I'm not sure.
A problem with this is that originally people say "Awesome, folders!" Eventually it will become, "Folders are too small, make them bigger nao please!" It might not happen, but it's very possible.