If the site does a lot of curation and 'good writing' then I would say no, since wikipedia is a little austere. For example, if I want to read about oddball geographic places then Atlas Obscura is a much more interesting place to read.
Yes. I was thinking in the lines of Karp's 21 NP-complete problems or Hilbert's problems. There is indeed a Lists of unsolved problems page in Wikipedia itself (https://en.wikipedia.org/wiki/Lists_of_unsolved_problems). That is why the Wikenigma site in the original post sounded a bit redundant, though nothing wrong with a wiki focused on just that.