How do we predict where the next MRT lines will be? Is it true that property around MRT stations will have higher rental yield?

LTA master plan is a good place to look. They have already planned until the year 2040! The TEL line is coming up soon, followed by the Jurong Regional Line, then the Cross Island Line.

Do you mean predicting BEYOND what is already shared as future plans?
https://www.sgtrains.com/network.html
Future Lines

"New rail lines have been planned to be built in the next 15 years. These pages will provide a brief overview of the line history and its proposed alignment. The actual alignment of these lines has not been finalised yet."
The Cross Island Line will be the eighth line to be introduced into the MRT system.

Designed to be a fully underground and automated high capacity rapid transit line, it will be Singapore's longest underground MRT line and will span about 50km with around 30 stations across the island.

The line will be opening in three stages from 2029, and will enhance east-west travel from Changi to Jurong Industrial Estate; serving as an alternative route for the East West and Downtown Lines.

The eastern leg of the line will also extend outwards from Pasir Ris to Punggol.