At the end of the latest CW/DC crossover, "Crisis on Earth-X," Barry Allen's arch-nemesis Eobard Thawne, who worked for the evil Nazi regime in that reality, was able to escape, with Barry's help. As Thawne disappeared back into the timestream, he suggested that this won't be the last time the two meet — but will Eobard Thawne return to The Flash in Season 4?
The network hasn't announced a specific time to expect more Reverse-Flash, but given that this villain is a master of time and inter-dimensional travel, he can basically appear anywhere at any time. And with his sharpened time-traveling skills, he might already know the time and place of his next clash with Team Flash. He even suggested that he might be adopting a whole new identity the next time they battle, quipping, "I wonder what face I’ll be wearing next time we meet, Flash."
There may be a small hint embedded in a comment Flash star Tom Cavanagh — whose many characters include the version of Thawne with Harrison Wells' face — made just before The Flash Season 4 began. The actor told Comic Book Resources at San Diego Comic-Con that Wells could go through another transformation.
"This year, we’ve had a different villain and so I think there’s going to be areas that we could use a different Wells," he said, likely referencing the Thinker, The Flash Season 4's biggest villain so far. "We're going to start with Harry and then find a way, I think, to try and get a different version of Wells in there, be it some form of version of Harry or whatnot."
Now, this could just be a reference to Thawne's temporary return during "Crisis on Earth-X," or even the "Council of Wells," but Cavanagh followed this up with another statement that suggests a slightly different possibility. "There is stuff that's involved with the season, how Wells might become a different version of Wells, but I think that's a storyline too far for me to give up," he said. The key here is that he's implying that Harry might go through some kind of transformation — "how Wells might become a different version of Wells."
Thawne has already killed one Harrison Wells and pretended to be him. Could he possibly pull the same trick again? It certainly seems unlikely that Team Flash wouldn't notice that their resident Earth-2 super-genius was replaced with a doppelgänger. It could also be possible that Cavanagh simply meant that Harry would be replaced with another version of Wells, the way H.R. stepped in for the last season as the resident fedora-wearing BS artist who nobly sacrificed himself to save Iris at the end of Season 3. It seems too far-fetched to imagine Thawne joining Team Flash now, as probably the most powerful super villain Barry's ever battled, but the series has rehabilitated bad guys before (and The CW's DC Universe is full of villains turned heroes).
There is another possibility, and that's that Thawne could be involved in flashbacks. Thawne, while disguised as Wells, was responsible for the creation of DeVoe's powers. This has already been revealed through flashbacks, so perhaps the series will give more context to the Thinker's origins. Or, Barry could potentially try to mess with the timeline in order to stop the Thinker from even receiving his powers in the first place. You'd think post-Flashpoint that he'd think twice before attempting to change the past, but maybe DeVoe will be so powerful that Team Flash doesn't see much of a choice. As a fellow time traveler, Thawne could wind up being an unlikely ally, especially if he's threatened by the idea of a man so much more intelligent than he is.
However Thawne winds up returning, it seems likely from Tom Cavanagh's comments alone that he'll never be totally gone from the show, even as more versions of Wells populate the team. ("My starting point is always the Reverse Flash," Cavanagh told CBR in the same interview.) So fans would be smart to keep Thawne's final words to Barry in mind as the season continues.