I think that's a superfluous feature. Priorities in Dayboard are binary - it's either important (thus it goes on your 5 tasks) or it's not.
You can sort your tasks visually though. So if it helps to have the 5 tasks ordered by importance, you can already do that now!
Yeah, a common request is the "future list" but I'm against it because it encourages people to spend time "organizing and planning" inside Dayboard -- which 1) is what most task management / to do lists already offer (and isn't necessarily doing a bad job at) and 2) isn't the actual work that people should be doing. Having no more room to add more tasks means it forces you to move on to actually do them (a problem, that perhaps you might not have but many others would).
But anyway, I might change my mind so never say never. :) Thanks again for the suggestions!