There is a new official trailer out. By what it shows I could imagine that the steep turns (don't know how they are called in English) in Monza are to be released next. I am no Monza fan myself, I'd prefer to get Mugello and/or Nürburg first.
Since it is announced as a separate DLC (free for early access customers), I fear my beloved Spa comes last.
No track there is that I have collected more rounds on.