According to the "leaker" ShrimpApplePro, the screens of the next iPhone 15 Pro will be more efficient, with that of the Pro Max even brighter than the current versions.
2,500 nits on the screens of the iPhone 15 Pro Max?
Apple would thus adopt the next generation of Samsung Display screens capable of a peak brightness of 2,500 nits, compared to 2,000 nits for that of the iPhone 14 Pro Max, which is already more than satisfactory. Again, this would be a feature reserved for the premium Pro Max model, with the even higher-end 'Ultra' version apparently slated for 2024 with the iPhone 16s. September of this year.
More details will be known from June when the new features of the next iOS 17 will be revealed.