There would be no Singapore.
If we use the 1824 Anglo-Dutch treaty as a POD, there already was a Singapore. That is the most likely reason why the British wanted Malaysia, since I don't think Penang would be enough reason.
Mind you the treaty did not give all of Sumatra to the Dutch. They only gave Sumatra south of "I am not 100% sure". Northern Sumatra fell officialy in the British sphere of influence. When the Netherlands sold the Gold Coast forts to Britain they also included in the treaty that the rest of Sumatra would be part of the Dutch sphere of influence, which marked the start of the attempts to capture aceh by the Dutch.
So if the treaty was different and the Dutch would gain British Penang, while the British got Dutch Sumatra. I think the Dutch would get short changed in this deal. Especialy if the British keep Singapore. With Singapore they already control the strait of Malakka. Now they even got Sumatra, thus also control the Sunds strait.