The stupid amount of escalation required every new season especially in genre fiction is what makes this shit unbelievable anyway. I mean they literally went from fighting crime bosses to vengeful ex-friend mercenary to ninja clan and now it's magic dude with an army who plans to nuke the world. Dat escalation. Same with Flash too.You would have to completely change the focus of Arrow if this happens. The current season would make even less sense if Supergirl could just swoop in, fly Dahrk to the fortress of solitude and zip out.
Also I would wager a guess but if Supergirl was originally picked up at the CW I would imagine LoT might not even exist in its current capacity and would actually star all 3 leads and other heroes doing cool shit instead of what we have now? I dunno.