Tom Cavanagh is the master of playing more than one character in the Arrow-verse. He's gone from Wells to Wells to Wells in the five full seasons so far, not to mention Reverse-Flash at different points in his timeline. That said, he's generally only played versions of Wells and then the supervillain hiding behind the face of a Wells. The upcoming "Crisis on Infinite Earths" crossover that promises to rock the Arrow-verse in some lasting ways will see Cavanagh tackle a new role, and this is a huge one from the comics.
Yes, Tom Cavanagh will be playing a comics character by the name of Pariah. At the Flash panel at San Diego Comic-Con 2019, new showrunner Eric Wallace dropped the news and shared some tantalizing info:
He will also be playing Pariah as part of the crossover. For folks who don’t know, in the original miniseries, Pariah, who was very much integral to releasing the Anti-Monitor onto our multiverse, which then more bad things happen. For Tom to play such a pivotal character, not only is he psyched, but I think his interpretation that is going to… just set the bar.
The Monitor has been making efforts to prevent the crisis from "Crisis on Infinite Earths" from destroying everything, and Pariah should be a big thorn in his side. The person who catches the attention of the Anti-Monitor -- even unintentionally, which is the case of Pariah in the comics -- will unleash a lot of destruction and potential ruin on the multi-verse. So, he has that going for him!
At this point, I hardly have any guesses about how and why Tom Cavanagh will play Pariah. Is Pariah a distant descendent of a Harrison Wells, and it turns out that Wells' genes are strong enough that future men in his line all happen to look like him? Is this a body swap or body transformation or even hologram situation? The Pariah of the comics doesn't exactly look like Wells:
Of course, considering that the Arrow-verse's long-awaited take on "Crisis on Infinite Earths" is still months away, fans dying for spoilers can always read the legendary DC Comics miniseries. There are inevitable differences, as some of the key players in the comics run were never introduced in the Arrow-verse, even if Themyscira does exist and Batman is somewhere.
Hey, at least the Arrow-verse is getting a double dose of Superman! Eric Wallace revealed at the panel that the case involving new Flash villain Bloodwork, who is only one big bad of Season 6, will apparently be a very big deal, if his arc somehow launches "Crisis on Infinite Earths." Here's how Wallace put it:
The secrets of why Tom becomes Pariah are actually part of graphic novel #1. And all lead up to a very, very big twist at the end of the Bloodwork case that launches 'Crisis.'
Tom Cavanagh presumably won't be playing Pariah just because the Flash producers wanted to include him but couldn't find a way for one of his existing characters to participate. Whatever version of Wells he plays this season may not need to be involved in the crossover, considering it will involve characters from five shows; Reverse-Flash probably will.
The Flash ended Season 5 on a tease that Reverse-Flash has a part to play in "Crisis on Infinite Earths." While Eobard Thawne has been played by another actor, such as when he moved to Legends of Tomorrow for a villain-packed season, I doubt the show would have used Tom Cavanagh's Thawne to tease crossover involvement and then switch actors so Cavanagh could randomly play Pariah. There could be infinite variations of characters in "Crisis on Infinite Earths," so we probably shouldn't rule anything out at this point.