7) Killer Frost (Earth-1)
Fans were always expecting Caitlin Snow to turn into Killer Frost as per the comics, but season two of The Flash subverted this expectation by making the character the Earth-2 version of Caitlin. Now that the character is dead, however, we think it might be time for Dr. Snow to take on the identity herself.
Killer Frost is a pretty major DC villain, so it would be very surprising if The Flash had washed its hands of her already. It would also make perfect sense for Caitlin to snap, after the terrible experiences Zoom put her through in the second half of season two.
Alternatively, Killer Frost has had multiple iterations in the comics, so the character’s return doesn’t have to involve Caitlin. The original version of her was Crystal Frost (who was actually in love with Martin Stein). Perhaps this could be Caitlin’s mother that we’ve heard was a cold and unfeeling parent?