While most details remain sketchy, the teaser that surfaced online earlier this week suggests that the Galaxy M31 will hit the market with a 64 MP main camera and a generous battery able to provide at least 5,000 mAh like the Galaxy M30, maybe even more than 6,000 mAh (Galaxy M30s).
https://www.notebookcheck.net/Samsung-India-teases-upcoming-Galaxy-M31-64-MP-camera-and-a-generous-battery-in-tow.453044.0.html