Tossing in spoiler tag as precaution
1. Hunter Zolomon
2. earth 2 Henry Allen, possible but not likely. John Wesley Shipp did state in an interview that he really did not want to wear the hot and uncomfortable costume again. But he could have been joking and money does talk.
3. Earth 2 Barry Allen. An even greater threat then Reverse Flash would be Barry himself
4. Earth 2 Harrison Wells, possible, or he may become an ally to Earth 1
5. Someone we have not met yet that may not be Hunter Zolomon
Think that barry or eveil father would fit the bill nicely, and the Flash team just might need to recruit the 'good" dr wells in order to stop him!