Hmm, it's a good idea, but I think it would be easier to achieve (considering coding) with another function:
We already have "start paused download when a download finishes". What we are missing is a feature I know from emule that's "continue file from same category if possible". By implementing this option you could nearly achieve what you are talking about.
OK, it is not really capable of what you want to do... Your idea is quite interesting, too, but a little harder to implement (or at least it takes more work to achieve it).