Andrew Kreisberg just finished up his Winter TCA Panel, touching on the news that Kevin Smith (Mallrats, Dogma) would be delivering an episode of The Flash. He added that just because Smith and Mark Hamill are good friends, that doesn't mean The Trickster will be returning in Smith's episode. When asked about the return of another villain, Kreisberg stated that Grodd won't be back this season but that doesn't mean he's gone for good. In fact, Kreisberg's comments could be taken to mean that Grodd will become a primary antagonist for The Flash in a future season.
"I would liken the Gorilla City thing to Ra’s al Ghul. You know, you hear about “the man from Nanda Par Bat” in [Arrow] Season 1, and “the man who showed Malcolm the way,” and then you met Nyssa, then in Season 3 you met Ra’s. So, it’s a long-term thing."
The Flash returns from mid-season hiatus on Januray 18th at 8/7c on The CW.