Indian balkanization is a lot less likely than a lot of people assume.
I've said this before, but there's a lot of misunderstanding on this board about the persistence of states, about the ability of multiethnic states to endure, and about the nature and likelihood of balkanization. Most countries in the world are multiethnic or multilingual. It's kind of a European conceit to see only homogenous ethno-states as "natural" states. The only place you find these states is pretty much in Europe (and that largely as a result of the world wars) and Korea and Japan.
Moreover, countries don't just spontaneously combust. Colonial boundaries have mostly held, even in even more artificial or disparate nations like Indonesia or the Philippines. Countries tend to be sticky. Westphalian norms and the current international regime mean that the state system tends to prop up and support existing states. Administrative ties, economics, and national identities all help tie countries together. Plenty of countries experience civil wars or revolts, but they usually hold together even so.
The main examples of balkanization tend to be grouped into a few categories:
(1) The slow breakup of the Ottoman Empire - as a result of European interventions and the Balkan Wars;
(2) The reorganization of Central Europe after WWI, as a result of the breakup of Austria-Hungary and the old German Empire;
(3) The post-Communist collapses.
The first two are examples of countries falling apart due to major global wars, with the winning parties and outside powers helping to enforce new states. The second is an example of the particular nature of Leninist "unions." The USSR, Czechoslovakia, and Yugoslavia were all -- on paper -- very loose confederations, in which the republics were de jure sovereign entities with a right to secede written into their constitutions. These states functioned as centralized states so long as there was a one-party dictatorship, but once that party-system collapsed, the whole structure proved unworkable and the republics exercised their rights to secede.
Now, what does this mean for India?
It means that in all likelihood, whatever boundaries India had at independence would likely be its boundaries today. Had Partition not occurred everything wouldn't be sun and roses, but the result of continued communal tensions would likely be simmering insurgencies, political polarization, and possibly a breakdown into authoritarianism, rather than full-on collapse.
But it also means that the best way for a "balkanized" India is for it to be broken into smaller units by the British themselves prior to independence.
This isn't as likely as it may sound. British considerations were very much in keeping with a unified India. The Brits were hoping that a unified (but federated) structure would allow them to maintain a strategic alliance with the country, similar to their protectorates over Egypt and Iraq before the 1950s. That said, if it became clear that the Indian leadership wouldn't be a British ally, that consideration might have changed.
As it was, the Brits in the early 40s toyed with the idea of granting independence or dominion status to whichever provinces wanted independence. In other words, independence and unity would have been an opt-in situation. Nehru angrily denounced the idea, and the Brits were ultimately in no position after the war to hold onto any part of India.
But in a situation where World War II is arrested early or never occurs, a stronger Britain may be able to hold onto India, or at least pieces of it, into the 1950s or even 1960s.
It isn't inconceivable that under the 1935 Government of India Act, you ultimately get the major provinces, and a good chunk of the princely states, agreeing to federate into a "Union of India." But if the British are in a stronger position, they may be willing to continue ruling the Muslim provinces, and continue their protectorates over some of the larger princely states, like Hyderabad and Kashmir. An independent Tamil Nadu is possible but unlikely, given the Justice Party's decline in the 1930s, as Indian nationalism rose. But if a separate Tamil province had been carved out earlier, maybe it's possible.
The end result wouldn't be quite a "balkanized" India. But it could resemble the current map but with somewhat different frontiers. Perhaps a "Pakistan analogue" of a united Punjab along with Sindh and the Northwest Frontier Province ultimately getting independence either as separate countries or as a separate federation; an independent Baluchistan and Kashmir; an independent Hyderabad; an independent Goa; maybe a couple of other independent princely states like Bhopal, and a separate East Bengal (ATL Bangladesh).