From PSLS
Nomura didn't detail what this paid DLC would include, but fans shouldn't expect multiple releases throughout the year. "For paid DLC, we want to combine a lot of things and release at once," he said. While he did mention the content "will take time," he also did say "we want to finish working on the DLC by this year." So expect an announcement and release by the end of 2019.
Nomura did confirm that Critical Mode, which has been highly-requested, will come via a free update. The extra-hard difficulty mode is still being tweaked, but it will be ready sometime in 2019, at least. Other free updates will be coming throughout the year, as well, though we don't have a clear idea on what those will be quite yet.
It was confirmed that this DLC is taking the place of a "Final Mix" re-release that previous Kingdom Hearts games have gotten.