During the second triumvirate, Mark Antony was supposed to be in charge of Rome's Eastern provinces and Transalpine Gual. However Octavian was able to seize control of it and its 13 legions. What would happen if he was unable to do so or he never tried?
Didn't Antony get most of the Gallic legions returned to him in exchange for letting Octavian keep the province? IIRC, Octavian got to keep two of the Gallic legions since Antony already owed him two legions under their agreement after Philippi (Octavian had given Antony two of his legions in Macedonia in exchange for two of Antony's legions in Italy, but when Octavian returned to Italy Lucius Antonius and Fulvia reneged on the agreement and refused to hand over the two legions), and Octavian had already given four of the Gallic legions to Lepidus to help him secure Africa, but the rest of the Gallic legions were returned to Antony as part of the Treaty of Brundisium.
Anyway, if Antony never loses Transalpine Gaul then that has ramifications for control of the African provinces. IOTL Octavian gave Lepidus six legions (two that had defected from Lucius Munatius Plancus during the Perusine War, and four of the legions that Octavian got from Gaul) to secure Africa. That gave Lepidus enough troops that the governor in Africa, Titus Sextius (an Antony ally), didn't try to oppose him and turned over the African provinces (and their four legions) to Lepidus without a fight. If Octavian never has control of the Gallic legions though then he can't grant four of them to Lepidus which means Lepidus doesn't have enough troops to make Sextius give up Africa if he doesn't want to do so. Now Antony might yield Africa to Lepidus anyway since the understanding after Philippi was that Lepidus would be given the African provinces (if Octavian was satisfied Lepidus hadn't been double dealing against Octavian and Antony while they were fighting Brutus and Cassius), but if Antony does decide to yield the African provinces to Lepidus, he is probably going to insist on at least keeping the four African legions (since with his Gallic legions still needed in Gaul, Antony is going to need extra troops from somewhere to fight the Parthians.)
I don't know how this will effect Octavian's war with Sextus Pompey. He won't be able to get nearly as much support from Lepidus in the war since even if Lepidus has the African provinces he is down 8 legions, but I don't know if Lepidus really contributed all that much to the ultimate victory over Sextus anyway. I don't know how important having access to the resources of Transalpine Gaul was to Octavian in being able to afford the fleet he built up to fight Sextus.
Assuming Octavian (or more accurately Agrippa) does eventually beat Sextus Pompey, Octavian will be facing much longer odds in any subsequent conflict with Antony, since Antony will have a bigger army than he did IOTL (he'll have at least 8 legions more than IOTL, since he'll keep the 4 Gallic legions that Octavian gave to Lepidus and the 4 African legions, and he also has the possibility of recruiting more troops from Gaul.) And of course a large Antonian army in Gaul is a dagger pointed right at Italy which will make it very difficult for Octavian to counter any move by Antony into Greece. Of course I doubt Octavian would risk war with Antony under these circumstances.