So I've decided a tentative timeline after a few discussions with
@Freedom2018. Indo-Iranians establish a large Empire in the core region of which westernmost is Zargos and Easternmost is Sogdia. Northernmost is Samarkand and Southernmost is Haryana/Delhi region initially.
IVC people are colloborated with and eventually form a good chunk of the Empire and Indo-Iranians and IVC are almost like one united peoples. Even the Pre-Iranian peoples in Iran like Gutians,Elamites,Caucasians join as they did OTL with Iranian peoples.
Now this Empire resembles Roman Empire as a multi ethnic and universalist empire. Religion is a combination of Indo-Iranian Paganism/Historical Vedic religion,Zoroastrnism,IVC 2.0 religion.
Now slowly Anatolians,Hurrians,Urartians,Armenians and Greeks start to get influenced and start influencing this empire and they will form a large population in the Western regions.
Tocharians also come in and form a good percent on the Eastern regions. Northern regions is mainly Scythian and Sogdian dominated with the ruling classes initially of people of Western Iranian and Indo-Aryan origin who would be closer in this timeline. Eventually Scythians are taken in like how Gauls and other peoples were taken in by the expanding Roman Empire.
Central and South India OTL will be a Unity in Diversity with many extra languages,ethnicities,religions living there initially.
Eventually a decentralised and a different type of Hybrid Christianity like religion is born which probably evolves from the Anatolian society rather in the Hebrew society in the Roman Empire. So you will have two types of Christianities in this timeline. The Western and the Eastern regions are European with Anatolians/Caucasians/Galatians and the Tocharians respectively and so will be the Northernmost regions with Sogdians and Scythians. The regions middle looks as you said,like Iranians,Pashtuns,Tajiks,Kurds,etc. The region beyond Maharashtra will be diverse with all these people living in different proportions and along with AASI peoples,Mixed Ethnicities and even European immigrants who are of all European,Iranian,IVC,Caucasians,Tibetian/Asian,Finnic and Ugric,AASI peoples,Dravidians,Anatolian,Semitic and Egyptian Ethnicities as it would be a fairly empty land initially and the Empires would bring them to settle as did Roman Empire. Though populated by all these peoples,the largest part would be like OTL North and Central Indians as far as this region is concerned. Europe of the Antiquity and Medieval eras would be quite different. Islam probably wouldn't rise and instead,the Arabs settle inside and join this Empire. Tibet too would look different and diverse. All follow the version of Hybrid Christianity like religion this Empire founded from the Anatolian society.
Rome and Greece have been deeply influenced during their evolution and before they expanding from these peoples and hence a closer relation is shared between Graeco-Roman and this empire. Both have been deeply influenced by each other for centuries during their rise.
All these peoples languages would survive making it the most diverse,expansive and the powerful country in Asia and one of the in the World. All Colors of peoples,all languages speakers would be united under a banner of a Roman equivalent Empire.
Further timeline will be continued after a feedback. We will discuss the religions part in detail.