5) A Crisis On Infinite Earths
One of the biggest questions fans have been asking ever since it was revealed that Supergirl is coming to The CW is whether the hero will remain in her own separate world or somehow be brought into the one that Green Arrow and The Flash call home. The latter option would certainly make future crossovers easier, and this four night event being about combining these realities would actually work really well as it gives The CW time to wrap up the stories started by Supergirl season one and then have her in new world after the Winter hiatus.
To make this a possibility, some sort of loose adaptation of A Crisis on Infinite Earths would be perfect. Both Supergirl and The Flash played a key role in that story, and having the Anti-Monitor destroy Kara’s Earth or somehow end up combining it with Earth-1 would solve a lot of problems and be truly epic for fans to see.