Today, some interesting geography and cultures from South America that might have existed in a similar way in Sundaland.
In Bolivia there is a watery plane of hundreds of thousands of square kilometres in size that is flooded for large parts of the year. In this area there are the remains of mounds connected by causeways, raised strips of land, artificial islands and canals. This type of environment might have existed in central Sundaland and a similar culture would fit right in.
Archaeologist Clark Erickson summarized the early Spanish description of Baure villages:
As usual I recommend you read the Wikipedia entry to learn more: Llanos de Moxos
Nearby in Venezuela there exist huge tabletop mountains called Tepui which function as ecological islands. These mountains served as the inspiration for the Lost World by Sir Arthur Conan Doyle, in which explorers discover a land of where Dinosaurs still exist.
Perhaps there exist hidden mountains, valleys and giant caves and sinkholes in what is now Borneo or Papua New Guinea where similar ancient animals, and perhaps other surprises, can still be found?