Since we got a real-world circuit last month, I'm guessing it's an original. Deep Forest is one we know that they've been working on since release; I could also see Matterhorn circuits come back since they were just created in GT6 and possibly done with GTS in mind.
If it's a real-word track, I don't think it will be an Asian track, as I'm pretty sure Kaz recently said we'd get some non-Asian tracks before another Asian track was added. Ascari and Silverstone are good guesses, they were also just added in GT6. Despite the meme, Spa is a contender as soon as the licensing issues are resolved, since it's ready to go. I could also see an American track being added due to the relative lack of American tracks in the game, so a returning American track like Laguna Seca, Daytona, or Indianapolis is possible.
One other note: I don't think we'll get another circuit that's new to the game, as even though it's been almost 2 months since we got Autopolis, Autopolis was brand new to the game. As a result, much as I think COTA is likely to happen, it won't be this update.