Let's list out potential Augustus succession PoDs I can think of.
Potential successors are - Marcellus, Gaius, Lucius, Drusus the Elder and then Tiberius. I am ignoring the ones like Agrippa Posthumus that fell out of Augustus's favor. Given the kind of shrewd and astute politician he was, if someone fell out of his favor in matters of succession it was probably for good cause. (Augustus Fan-boy alert

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First to die was Marcellus, his nephew. While Marcellus was a good choice and he almost succeeded Augustus when the latter almost died of a sudden illness in 23BC, it wouldn't have been a smooth succession. The senators weren't as thoroughly cowed as they would be in about say a decade and half later. He was however young and unproven at that time and moreover his succession would've raised too many eyebrows about the monarchic nature of the succession. So his succession was going to be problematic even later on, that is, if he had survived. He of course didn't and he died in the same year (23BC) presumably because Livia, Augustus's wife, had him poisoned.
It is possible that it was not Livia but someone else. Agrippa (Augustus's general) and Maecenas (Augustus's advisor) both did not favor Marcellus along side Livia. It will be quite difficult for Marcellus to consolidate his succession when the three closest and most influential people in Augustus's life where antagonistic to his succession.
Then came Lucius. He fell 'suddenly and gravely' ill on August 20th 4CE. Possibly the handiwork of Livia.
And two years later followed Gaius who died from injuries sustained in battle in Armenia on February 21st 4CE.
If Gaius and Lucius survived they would've made a formidable duo to succeed Augustus. By all accounts Gaius was competent and was favored by Augustus to succeed. It was his death that paved the way for Tiberius.
Finally there was Drussus the Elder. He literally fell off a horse and died in the spring of 9BC in Germany. If he had survived, Germania up till the Elbe/Albis was going to be Roman land. That was his conquest and his older brother Tiberius adored him. If Drussus was around, the whole Teutoburg Wald debacle would not have happened. And Tiberius or Drussus, whoever succeeded Augustus would definitely at the very least hold on to Germania Superior.
I sometimes wonder if Western Germany was turned into a Roman province and settled by wave after wave of Roman veteran legionnaires who gradually assimilated the local Germanic tribes in the Roman towns then how different would the history of Rome would've been. It would've definitely been a well developed and fortified buffer to the Migratiory tribes later on. The tribes would have to cross this land before they could reach the heart of the empire in Gaul and Italia. Not to mention many of the major tribes that migrated into and settled in Roman territory originiated from the Western Part of Germany.
A wild card successor would be Marcus Vipsanius Agrippa becoming the next Emperor in the event of the sudden death of Augustus. Sometime between 23BC when Marcellus died and 5 BC when Gaius and Lucius, children of Agrippa where designated as successors of Augustus, provided Agrippa survived beyond 12BC when he died in OTL. Perhaps if he lived for a decade more, Livia wouldn't have dared to plot against Gaius and Lucius.
If Agrippa and his sons with Augustus's sister Gaius and Lucius outlived Augustus then certainly Tiberius and Drussus would've been sidelined in the raced to succeed Augustus.