Greater India certainly would be interesting.
If they can stabilize things quick enough, but still go the same course as OTL, IE closing itself off economically for a few decades before
opening up in the late 70's-early 80's it could make China much worse off economically, and itself much better off.
As for how big it could've been, I'd say basically the Raj + Bhutan, Nepal Sri Lanka and the various Indian ocean islands.
While having Afghanistan part of it is'nt gonna happen, atleast not for a very long time (as in mid 21st century), I could see them puppeting
it along with Tibet and possibly Thailand.
Oh, and 'Indus Maximus' would've been a cooler title.