Many channels that re-upload fanfics are unaware of this, but by copying and distributing works that are not their own, they are committing a crime.
By directly copying a fanfic and narrating it in any form without the author's permission, you are infringing on copyright, as you're using someone else's material to gain views or income. You must respect the author's rights over any fanfic.
For that reason, I urge all "What if" channels to avoid falling into illegality. Before uploading any video that includes a work that isn't yours, you must seek permission from the author. In your permission request, you should include a link to your channel and whether or not you profit in any way from their content. Additionally, in the videos you upload, you must include proof, preferably a public comment from the author (no photos, nor screenshots, etc.). If you do not comply, your channel may be shut down. There's a group called "Project Copy-Knight", which checks channels and verifies if they are violating copyright. If they find infringement, they notify the author, who then proceeds to file a complaint against your video(s), resulting in strikes against your channel.
Request Permission
Always ask the author for permission if you want to re-upload their content (whether written or in video), specifying what you plan to do with their work, and if there will be any monetization involved.
Don't try to bypass the system
Project Copy-Knight has an advanced system that will detect any matches between your video and the original work. Methods like paraphrasing or rewriting won't help you evade their system.
Follow the guidelines
If you want to learn more about how to avoid copyright infringement or if you have any questions, check out this page, which outlines the conditions to assess your channel and determine whether it can be subject to a complaint.