If the Johore Rhio rulers did not sell Singapore to the British, the Dutch would most probably have moved in and taken over the the whole of Malaya.
That being the case, Malaya would have been part of the Dutch East Indies and on independence, Malaya and Singapore would have been part of Indonesia. Hence the vision of Greater Indonesia of President Surkarno.
Imagine all of us as Indonesian citizens reporting to Batavia/Jakarta............