Judging by the splash made about the upcoming announcement (the official website even has a countdown-clock), this is likely to be a more substantial announcement than just a DLC-pack or similar.
With most major licenses that would warrant such a large-style announcement (Ferrari, Porsche etc.) already present in the sim, it’s unlikely that the announcement is licensing-related.
The most likely scenarios are either an announcement on the simulation joining the growing eSports-market or, even bigger, the announcement of Assetto Corsa 2, something that was hinted at in interviews last year.
Until the cat is finally let out of the bag on Wednesday, feel free to speculate away in the comments below.