Well, there is a reason /r/subredditoftheday is twice as large as /r/redditdayof. I think it is that the former introduces me to new classes of ideas instead of just throwing me a random subject. If I wanted random subjects, well, Wikipedia has a button for that. What makes Sub o/t Day better for me is that if exposes me to part of the website that I wouldn't find on my own.
So I'd suggest a weekly exposure of a less popular tag on hubski, eg "here's a less used tag, let's use it as a promt
to find articles this week". I would find it.more interesting than "let's talk about ducks for a week". It might just be me though.