Augustus die in 23? Augustus' heir will be without any doubt Marcellus if he is still alive and his death is butterflied (OTL he die in september of 23). If he is already dead and Augustus had not make already any other choice (Julia will remarry with Agrippa only in 21) or if he is still alive but die on the schedule then any new husband of Julia or Marcellus' brothers-in-law or Tiberius can have a possibility to grab the power. The most likely option (specially if we are talking of Marcellus' and not directly Augustus' heir) is Drusus (younger son of Livia, Augustus' favourite step-son and husband of Antonia Minor, Augustus' youngest and favourite niece) and the most likely alternative Agrippa (who at the time was still married with his second wife, Marcella Major (OTL he divorced from her to marry her cousin Julia), the oldest of Octavia's children)