I’d think the easiest thing would be to just write it as actually being Rose. In order to do x and switch realities she had to hijack The Doctor’s regeneration. Then you could just write her as Rose.
And that’s a totally reasonable story to write. The Doctor regeneration into a former companion is an interesting story beat. Why this character? Why now? Do they play the character as they were or as the other Doctor saw them? There is a lot you can do with this regeneration.
I’d think the easiest thing would be to just write it as actually being Rose. In order to do x and switch realities she had to hijack The Doctor’s regeneration. Then you could just write her as Rose.
And that’s a totally reasonable story to write. The Doctor regeneration into a former companion is an interesting story beat. Why this character? Why now? Do they play the character as they were or as the other Doctor saw them? There is a lot you can do with this regeneration.