You seem to be very knowledgeable about Mauryan Empire, could the Empire have been centralized like Qin China and Imposed Cultural integration and assimilation to create a Pan Indian Identity ?
The empire could have centralized yes, I'd argue that that some extent of imperial integration via the use of the Sangha could also have occured, acting as a support to the greater empire. But a panindic identity via assimilation (forced or otherwise)? Unlikely. Even China south of the Yangtze has remained full of distinct ethnic groups simply having to attach themselves to a greater Han super-identity and usually the south was the first to break apart unless imperial administrations moved there directly like during the Song or Ming dynasties.
What needs to happen is to make tributaries, feudatories and local populations view the structures of imperial state as vital to their benefit. The Mahajanapadas provide a nice analogue for the duchies of China, so rather than Mauryan rule diminishing them and subsequent rulers confining them to history, it might be more prudent to integrate them in some method to the rule of the empire.
A United Indian subcontinent - even one centered on Pataliputra is easily possible - but indefinitely? You're always going to be hit by invasions from the steppe, sooner or later.
And the geography of India is simply not conducive to a single unified empire with any degree of centralization. Too many geographic impediments, not enough Mediterranean or major river valleys. The Indo-Gangetic plain is easily unifiable - the rest, not so much. And the question thus becomes whether the Indus and Ganges can hold the whole subcontinent down. And history shows us they're not great at that, long term speaking.
Modern powers aside, no ancient empire could establish a geographically united India until the modern day, imo. Ashoka didn't give autonomy to distant regions because he wanted to.
I don't think this is necessarily true the 13th Rock Edict of Shoka actively discourages any intense involvement in re-conquest or conquest of lands:
"Ths inscription of Dhamma has been engraved so that any sons or great grandsons that I may have should not think of gaining newconquest."
As for the geography of India not being conducive to unity I agree to some extent but the repeated campaigns by later empires like that of Samudragupta, Harsha, Chandragupta II and others suggest that if given precedence of some sort of unity there could have been imperial rule from Pataliputra with feudatories in the south and north-west with regional administrations. But yes, south of the Vindhyas even the Aryan mahajanapadas like Ashmaka and Mutiba ruled ad hocover what were largely realms.
Geography becomes a lesser (but still vital) factor if you have good infrastructure to support large well-organized armies, which the Mauryans were trying to implement under Chandragupta and Bindusara, and an extensive bureaucracy, which started failing after Ashoka's rule due to the Sangha being given such power that it was a force unto itself rather than an appendage of the state. But the problem of the Mauryas is that they would fall sooner or later because of issues related to but differing from geography. It is likely the empire fell because the agrarian economy of the Indo-Gangetic basin couldn't support what was a largely non-agrarian empire south of the Vindhyas (aside from the Three Kingdoms of Tamil Country), no matter how many times the Mauryans debased their currency to energize it. As for invasions from the steppes... China had to deal with it more frequently and more often.* Once again its about setting precedent for the Sakas, Kushans, Indo-Greeks and Parthians to emulate the Mauryan rule. Menander of the Indo-Greek kingdom did try to emulate Ashoka and the Buddhist texts make a comparison between the two but it simply falls down to rotten succession in his case.
So the point of this thought exercise is to try and set precedent for later rulers to be willing to relinquish some territories but adhere to the dynastic cycle rather than try and establish new states.
*I do see the point you're trying to make here though. The north-west will be frequently lost and regained if a dynastic cycle is established, but the cultural influence of a united (read: cohesive to some degree) Sangha operating with the state might better integrate them to make it seem more like a doorway to-and-from the subcontinent, rather than simply being a gate into India.
