I was just reading through a couple of those old threads; in the discussion of one of them NYI suggested a potentially fantastic theme:
"Fixed it" - take an outfit you've already posted in the WAYWRN at least the week before (to prevent "gaming" by posting a deliberately bad first outfit during the challenge week), and improve it for your entry, submitting the old outfit alongside the new. It would be a great learning opportunity for all of us, though it would limit entries to those already posting in WAYWRN. To broaden entries, one could do it as a "Thought Process" challenge, where you post what you were going to wear and also what you actually wore, describing why you changed/improved a detail. Voters can decide for themselves how to vote (i.e. purely on absolute success of outfit, or most improved, or a blend).
I think that's brilliant. I like my suggestion of a "What would you wear to (x) event" challenge, and those suggestions above are even better.