Seleucus Nicator, accompanied by his Greek and Persian troops, invaded India around 306 BCE, with the ostensible goal of recapturing the Indian provinces of Alexander. He came into conflict with Chandragupta Maurya and his generals. The details of the conflict are lost, but the peace that followed gives the idea that he must have been defeated since the peace greatly favoured Chandragupta Maurya. Seleucus handed over provinces of Gandhara (roughly today's Kabul valley and the adjoining North West frontier provinces of Pakistan), Aria (today's Herat region and central Afghanistan), Arachosia (today's Kandahar area and southern Afgnahistan), and Gedrosia (roughly today's Baluchistan) in return for five hundred war elephants. This leads many to believe that Seleucus had the worst of the battle between the two kings.
Now here is the departure from OTL. Suppose, Seleucus is not just defeated, but he and his sons are also killed in battle against Chandragupta's troops. Chandragupta was a clever diplomat – he sees the abilities of the Iranian horsemen of Seleucus and simply co-opts them (particularly the cavalry) into his own army. When the emissaries of Cassander and Lysimachus come to invite Seleucus into the coalition against Antigonus Monophthalamus, there is no Seleucus. But they are desperate and they offer to Chandragupta the share offered to Seleucus – all territory east of the Euphrates in return for his help in destroying Antigonus (originally, Seleucus was limited to territory east of the Euphrates – when Ptolemy failed to join the coalition, his share - Phoenicia, Judaea, Syria and Cilicia - was mostly gobbled up by Seleucus). Chandragupta, seeing the danger of a victorious Antigonus trying to emulate Alexander and invading his territory agrees, and sends the force that roughly equals what Seleucus could have commanded to aid the anti-Antigonid coalition (in fact, Chandragupta would have been able to send more help to fight Antigonus, since his resources were greater than Seleucus' resources.). The battle of Ipsus occurs as in OTL – Antigonus is dead, and Chandragupta Maurya annexes all territory east of the Euphrates – he might even grab Syria as Seleucus did in OTL. Nevertheless, the challenge is this
Make the Mauryans hold this extensive empire intact for five centuries. Be realistic - no endless `great emperors' and no hand waving away all challenges of logistics and resources.
Now here is the departure from OTL. Suppose, Seleucus is not just defeated, but he and his sons are also killed in battle against Chandragupta's troops. Chandragupta was a clever diplomat – he sees the abilities of the Iranian horsemen of Seleucus and simply co-opts them (particularly the cavalry) into his own army. When the emissaries of Cassander and Lysimachus come to invite Seleucus into the coalition against Antigonus Monophthalamus, there is no Seleucus. But they are desperate and they offer to Chandragupta the share offered to Seleucus – all territory east of the Euphrates in return for his help in destroying Antigonus (originally, Seleucus was limited to territory east of the Euphrates – when Ptolemy failed to join the coalition, his share - Phoenicia, Judaea, Syria and Cilicia - was mostly gobbled up by Seleucus). Chandragupta, seeing the danger of a victorious Antigonus trying to emulate Alexander and invading his territory agrees, and sends the force that roughly equals what Seleucus could have commanded to aid the anti-Antigonid coalition (in fact, Chandragupta would have been able to send more help to fight Antigonus, since his resources were greater than Seleucus' resources.). The battle of Ipsus occurs as in OTL – Antigonus is dead, and Chandragupta Maurya annexes all territory east of the Euphrates – he might even grab Syria as Seleucus did in OTL. Nevertheless, the challenge is this
Make the Mauryans hold this extensive empire intact for five centuries. Be realistic - no endless `great emperors' and no hand waving away all challenges of logistics and resources.