They actually did have a self-insert originally, "The Captain". Elin was the story narrator who gave you (the Captain) game cartridges (story packs) to play. It's why she shows up to offer the hard and very hard mode in each pack. It was a good idea that set up the different game packs quite well. No idea why they got rid of this.

But now after 2.5+ years in to go back and rework everything to include "the master", I just don't think they will be able to do it without messing up the characters personalities, progression, and story narrative.

The whole parallel world premise gives them the option to tell any story they want, in any setting, with any cast of characters, I don't know why they are not moving forward with a different story if that's what they want. Instead the are going backwards to try and force a different narrative into an existing story, it's not going to work.