1) Supergirl
As of right now, we still don’t know whether Supergirl‘s arrival on The CW means that she’ll become part of Arrow and The Flash‘s reality or will remain separate. The latter makes crossovers tricky, but so too does attempting to incorporate her world into the one which has already been established over the past four years or so.
[zergpaid]No matter how that plays out, we know that an epic four night event will take place this Winter which unites all of these characters. However, I don’t think that should be it for Kara when it comes to her interactions with the team; Rip mentioned that he saw a Man of Steel fall, so he’s clearly travelled between Earths, and Supergirl making at least the one appearance during season 2 (like Stephen Amell did last year) would actually make perfect sense.