As I have now reached my 1000th post I thought I better make it a good one.
What if Seleucos Nicator was not assasinated on the shores of Thrace?
In 301 BCE he joined Lysimachus in Asia Minor, and at Ipsus Antigonus fell before their combined power. A new partition of the empire followed, by which Seleucos added to his kingdom Syria, and perhaps some regions of Asia Minor. The possession of Syria gave him an opening to the Mediterranean, and he immediately founded here the new city of Antioch upon the Orontes as his chief seat of government. His previous capital had been the city of Seleucia, which he had founded upon the Tigris and this continued to be the capital for the eastern satrapies. About 293 BCE he installed his son Antiochus there as viceroy, the vast extent of the empire seeming to require a double government.
The capture of Demetrius in 285 BCE added to Seleucos's prestige. The unpopularity of Lysimachus after the murder of Agathocles gave Seleucos an opportunity for removing his last rival. His intervention in the west was solicited by Ptolemy Keraunos, who, on the accession to the Egyptian throne of his brother Ptolemy II, had at first taken refuge with Lysimachus and then with Seleucos. War between Seleucos and Lysimachus broke out, and at the decisive battle of Corupedium in Lydia, Lysimachus fell in 281 BCE. Seleucos now held the whole of Alexander's conquests excepting Egypt in his hands, and moved to take possession of Macedonia and Thrace.
But when he landed at the shores of Thrace. He was assasinated by Ptolemy Keraunos who, fearing that he has become unstoppable, assasinated him while trying to claim the Thracian throne.
If Seleucos was not assasinated or if Ptolemy's plan became foiled and after claiming Thrace moved onto the weak and seperated Macedonia and Greece. Then moving on the Ptolemaic Kingdom. Would they become too powerfull for the Romans to defeat or even rise? What would happen to the Romans who havent even moved out of central Italy? Would they send aid to either side in the Punic Wars? Would the Romans even be able to become the power they became?