If you compare both Germanicus and Tiberius. Yes there is a very realistic chance he could have become emperor. In Wells The Roman Empire he argues that Tiberius was commander of Illyrian Legions and Germanicus had control of the Rhine Legions.

The Rhine legions we are told in Tactius had been in almost combat combat since the lost at Teutoburg in 9AD. Tacitus then goes to explain the relationship that Tiberius and Germanicus had. When I read it I saw it as Tiberius being careful, Wells argues the same.

Tiberius knew he did not has the gravitas and authority that Augustus had, he didn't win a civil war. But he had seen what the civil war had done and was sure to have known about the civil wars of Marius and Caesar so it is only sensible to think that he was wary of the way that Germanicus could have marched against him with Legions from the Rhine as Caesar had done with his Gallic Legions in the 40s BC.

Tacitus is a great read, his implied and not so implied comments on Tiberius are really intriguing to see the views of a high ranking Imperial Official in the 2ND Century.