Tetsuya Nomura, the director of the Kingdom Hearts series, has been interviewed by Dengeki PlayStation about Kingdom Hearts III. Here, Nomura confirmed that DLC for Kingdom Hearts III is currently in development. A critical mode difficulty level is being worked on, as well as some expansions to the story of the game. Specifically, Nomura was asked about the following plot point from Kingdom Hearts III, and the answer was that it will be explained with DLC.

Spoiler Under what circumstances was Xion revived, and how did she join the true Organization XIII?

It is unknown exactly when this DLC will be released, but Nomura stated that free additions may come "sooner" and be released as soon as they are ready, while, the paid DLC will be grouped into one single package and is aimed to be released by the end of 2019.

Update: In Kingdom Hearts III Ultimania (the official companion book for Kingdom Hearts III) it was revealed that "critical mode" will be released as one of the free DLCs. Thanks go to @khunion for this tidbit.

Nomura also talked about whether there are plans for the future of the series (answering that there aren't specific plans for now, but discussions will take place), mentioned that Kingdom Hearts Union χ[Cross] will soon be moving to a new world, and commenting on a couple of story points from Kingdom Hearts III.

We will update with a full interview translation as soon as possible.