As many of you know, in our reality Alexander the Great's empire was divided between Seleukos (the Seleucid Empire), Ptolemaios (Ptolemaic Kingdoms), Lysimachos, Kassandros (Greece and Macedonia), and etc. Let's change this. Most points for replacing Seleukos. You cannot kill them until who inherits what is decided.
Easy.
Iran was the catspaw of several of the Diadochoi in the years before the beginning of the Seleukid Era. Let's start with Triparadeisos. You had Peithon, who ran Media, and who was generally opposed to a collection of the Upper Satraps, in loose alliance with Peukestas, satrap of Persis. Once Eumenes went east, he seized control of the Upper Satrapy alliance and beat the living crap out of Peithon. When Antigonos followed him, he got Seleukos on-side, and was able to co-opt Peithon as well.
So, first easy step is a Eumenes victory in the central Iranian campaign of 318-316. If Eumenes even does something as simple as avoid battle instead of engaging Antigonos' army at Gabiene, Antigonos will be forced to withdraw and deal with the West. Eumenes can then crush Peithon at his leisure, as well as, potentially, Seleukos. I've written a TL on this on another site, although it needs refinement.
But in OTL Antigonos won, and then drove Seleukos out of his satrapy of Babylonia and into the clutches of Ptolemaios, setting up the second easy step. In Ptolemaios' war with Antigonos and Demetrios, Demetrios was caught at Gaza in 312 and utterly
crushed. He had to pull back to the north, and Ptolemaios seized control of Syria. This set up Seleukos' triumphant return to Babylon, where he reestablished his power. Here we have the second easy step: either give Demetrios the victory at Gaza or give Ptolemaios a more Pyrrhic victory, so he's unable to pursue. (You can also fiddle with the campaigns of the historian-general Hieronymos of Kardia against the Nabataians, which took place around the same time, to give Demetrios an edge and throw a bone to any classicists reading. Just sayin'.)
Seleukos set about conquering the Upper Satrapies and Media, but once Ptolemaios backed out of the war with Antigonos, Antigonos and Demetrios were able to concentrate on the east once more, leading to the extremely shadowy Babylonian War, in which, apparently, Demetrios nearly captured Babylon itself but was narrowly driven out by Seleukos. (We get this not from any Greek chroniclers but from the Babylonian king-lists, which are damaged and do not match up perfectly with Greek works.) Here's the third easy step: permit Demetrios and Antigonos to crush Seleukos in the Babylonian War and resume control of the East.
The fourth easy step is the Battle of Ipsos, of course, which could've gone either way, but Demetrios pulled an Antiochos III (or should that be the other way around, since Magnesia happened after Ipsos did?) and took his cavalry out of play. If Antigonos and Demetrios win at Ipsos, it doesn't screw the coalition forces by any means, but it sets up conditions that could very easily allow for a successful Antigonid counteroffensive.
Anyways Greece would likely be overrun by the Gaulish invasions a few years after Seleukos conquers the area, if he is not assassinated by Ptolemy Keraunos.
I'm not so terribly sure about that. Keraunos wasn't exactly anything special in terms of military prowess, and seems to have deliberately stirred up the frontier by raiding groups that otherwise might have kept the Gauls in check, possibly to shore up his own military prestige against weaker opponents. And Makedonia was clearly in some sort of political ferment against him. I can see Seleukos more or less leaving that stuff be.
What would be interesting about a Seleukid Empire that successfully added Greece is Central Asia. Historically, Seleukos and Antiochos I invested enormous amounts of time, effort, money, and colonists into the region. Antiochos himself was viceroy of the area for a long time under his father's rule. If they control Greece, would that necessarily happen? How would that affect the later secessionism that evinced itself under the Diodotoi? Do the Parni even become important at all, or conversely, do they carry all before them?