Who would still be alive if Barry had never become The Flash?
With Barry’s mother alive, he would have never become The Flash, therefore Eobard Thawne theoretically had no reason to travel back in time.
As for which surprises or easter eggs might show up on the series, well that sky’s the limit on that one.
In the comics, Barry’s decision altered the lives of both villains and heroes alike, including Batman, Superman, Green Lantern, Aquaman and Wonder Woman.