Lately, I've been considering writing a TL in which Seleucus I has Ptolemy Keraunos put to death after the Battle of Corupedium, perhaps after landing in Thrace in 281 B.C., butterflying away his murder. This essentially leaves Seleucus in possession of most of Alexander's empire and allows him to go on to Macedon and claim the kingship there unopposed. In OTL, he wished to retire after this and live out the rest of his days in his homeland, leaving the governance of Asia to Antiochus. Now, the way I see it, Seleucus is pushing eighty at this time (he was probably around 78 years old at the time of his murder in OTL), but considering the lifespans of the other Diadochi and their great resilience, I could see him living another six or seven years, dying at around 84 or 85.
A few thoughts I have for the TL:
-Brennus and the Galatians will probably attempt an invasion of Greece in 280 B.C., just as in OTL. However, Seleucus is going to be secure enough in his position to ally with the Thracians against them, unlike Ptolemy Keraunos in OTL. Plus given the strength of his forces, I think that he'll be able to defeat them. This means that (at least for the time being) Seleucus has just saved the asses of both Epirus and the Greeks, which I imagine will have some interesting effects.
-Antiochus will probably go to war with Ptolemy II while ruling in Asia for his father, as he's going to want to retake Coele-Syria. As the events of TTL will butterfly away the marriage of Ptolemy II and Arsinoe II (who had a great deal of influence over matters of policy), the war may go very differently than the First Syrian War and I can see Antiochus, given his strength at the time, defeating Ptolemy and possibly also invading Egypt to eliminate its threat to Syria. Perhaps this will also lead to a native revolt in Upper Egypt? I'm considering a resulting client kingdom being set up, with either a native or a Greek puppet pharaoh (or perhaps annexation, although I'm not sure how likely that would be).
-Antiochus may also be able to secure Anatolia due to the lack of distractions in the west from the Antigonids and the Ptolemies. If his successors are competent, at least initially, I imagine that the Seleucid Empire will be more secure and have the resources to deal with the coming Parni invasions more effectively (at least until the rise of Rome).
Anyway, these are just a few ideas. I'd like to hear what everyone thinks and if there are any other immediate butterflies or effects from this POD?