I disagree with this notion since none of the Paradox Pokémon have anything remotely to do with the Terastal phenomenon. Neither sets of Paradox Pokémon have any prominent crystalline features (unless you think those green things on Iron Thorns are crystals). I imagine that any potential Terastal forms would have more crystalline features, possibly either in the form of special armor or with palanquin to accompany their hats/crowns.

Of course they could always just give the DLC Paradox Pokémon more crystalline features, but I doubt we'll go that direction since they seem pretty set on the whole past/future aesthetic instead.

That's a given for any regional battle mechanic nowadays; doesn't stop Game Freak from making one-and-done designs though, especially when it comes to marketing. What else could Game Freak do with Terastal in a Terastal-centric DLC anyway?