Can't say I found Malaysian Borneo that appealing from a beach standpoint.

Ambon and surrounding islands are worth looking into if you really want to get off the beaten track.

Koh Tao used to have a lot of appeal, but that was years ago. I don't know the present situation.

EDIT: I confused my Indonesian islands: I meant to refer to Ternate and Tidore. (Ambon when I was there several years ago was in the midst of ethnic strife and not a good place to be, though yes it has some very fine and deserted beaches.)