It's been brought up ad infinitum that any real union of Britain with an Indian colony would have to amount to India swallowing Great Britain, but I don't recall that ever being explored in any serious way. Combine that with a fairly vague idea of Anglo/Indian relations before the 20th century aside from the Mutiny and its immediate surroundings and I am left wondering, can anyone imagine a reasonably plausible way in which a politically unified, Indian led empire emerges at some point out of OTL's British empire that encompasses at least the Indian subcontinent and Great Britain?
It doesn't strictly have to be pre 20th I suppose, but this seems, off hand, most likely to come out of either (or both of) a very early PoD or from events surrounding the Mutiny, so this is where I put it.