Just for the assignments: I see what you mean, it is more of a challenge and the time to be online isn't important.
I still think that the matches might be better. So how about sending the requests (anonymously) to each writer and they have to pick two or three, so that it isn't dependend to be online? There still might be some requests which don't get a claim, but then it seems that those are difficult for everyone to write, no matter whether being assigned to or not.
It's not that I'm uncomfortable with the system as it is, but I think it might improve the stories if you think to have just some ideas for specific requests.