Actually upon further thought it probably is possible for a non-Javanese power to unite* the archipelago, but it depends very much on Java being very disunited for a long period of time. Pre and post European interference Java and Nusantara in general often trended towards disunity, due to the geography of mountains, jungles, islands, and sufficient fertility to support centres of power in lots of places. This is a problem for creating unitary states, but the examples of Majapahit and Srivijaya show that the way to do it is through a system of vassal states with considerable internal autonomy. It's not hard to imagine that if Java is disunited that one of several states, be it Brunei, Banten (my favourite), or perhaps a Malayan state may well over the course of the 17th and 18th centuries establish such a hegemony. Java being disunited is of course quite easy, just take the example of late Mataram and its successors, and keeping it that way is something that anyone could do with enough skill, such as how the Dutch did it in OTL.
The challenge is getting from there to a united state. By 'unite' I mean the creation of a single nation state, in the way that Italy or Germany or, obviously, Indonesia was created IOTL. The latter example was highly contingent on the Dutch domination, the shared experience of oppression combined with the very modern War of Independence, and by modern I mean with modern ideas that help build nations such as ethnonationalism and egalitarianism. A slower unification based on the gradual integration of an economic/military hegemony is a tougher thing I think, especially given that while for the sake of the scenario we are assuming "no Europeans" this in practice is highly unlikely, or at least there'll almost certainly be Indians and Chinese and some others wanting a piece of the pie. Resisting this would not break the laws of physics so it's not ASB, but would need to be the product of some very clever statespeople. Language at least shouldn't be an issue if Malay is turned into an even more universal language, of education as well, and religion may well help too. IMO there'd probably need to be a fairly successful 75-100 year stretch to turn the archipelago into a single (federal) nation state, but that's not impossible. If it included Malaya and the non-Indonesian parts of the archipelago it would have incredible potential, greater than all but a few continental states.