Ranked weekly challenges should be removed altogether. Ranked is for people who want to play ranked, not for people who want to just run around a bit just that they can cash in their “play 4 ranked matches” weekly challenge.
Instead, introduce a small XP bonus for winning a ranked match, for example 100 XP instead of 50. It encourages the very thing ranked is about: winning. Meanwhile, AFK’ers won’t be encouraged to join ranked, because in ranked, teams that have 1 or more AFK’ers usually won’t win anyway and thus don’t get the XP bonus.
I personally hate playing ranked because I don’t like the sweat fest and am probably just still too used to how easy Halo 3 / Reach PvP was compared to Infinite. I will admit that I actually have played and/or left ranked matches just so that I can be useless and get my challenges done in the process. In my view, the aforementioned changes would basically fix ranked.