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So to start, this is not going to be a total rewrite. This is just going to be little notes of things that could change because of the titular Companion from the Omniverse(mainly because I am not confident enough to do sweeping rewrites). This Companion is going to be a character I have had for about 3 years now, Sati Keyari. The gist of her character is that she is Ben's successor after he passed away from old age when she was 10. Many people knew that great dangers could rise again without the Galactic Guardian, so Azmuth, along with Professor Paradox, decided to find someone who could succeed Ben. Paradox, using his abilities, suggests Sati because he has seen her do great things just like Ben. So, to make sure that she is able to use the Omnitrix properly and be able to become herself again, and not Ben when she goes back to being human, she is whisked to Primus by a group of Plumbers to have her DNA become the human DNA in the Codon Stream, because Azmuth wasn't going to trust a 10 year old with the full Omnitrix, so he remade the Prototype Omnitrix and had to use the Primus DNA.
However, during the DNA preparation, Vilgax shows up, using Ben being gone as the perfect opportunity to finally achieve the goal that he had ever since 1776. In the scuffle that ensues between the Plumbers and Vilgax's forces, Sati is knocked into the Codon Stream. With her own DNA partially being in the Stream, it binds to her DNA, allowing her the abilities of the Omnitrix, without using the Omnitrix itself, which presented it's own sets of drawbacks. Another note is that parts of Ben's own DNA leaked into hers, giving her some characteristics that he had, such as her left eye twitching upon her lying, a peanut allergy, and a hunger for chili fries and smoothies.
Over the next 9 years of her life Sati did the job of being the Galactic Guardian, rose through Plumber ranks, and read through logs of all of Ben's stories, including the Ink Tank's 5YL, which is canon to this story, that is going to be important. On a regular trip to the Null Void, something went wrong with the projector, and instead of returning to her Earth, she ended up in The Doctor's universe, which didn't go unnoticed by either the Doctor or Professor Paradox. This is why, in this story, at the end of his second meeting with Rose, he rushes off suddenly. Paradox and the Doctor reach Sati at the same time and a conversation ensues with Sati wanting to stay and help the Doctor. So, the Doctor lets her onboard as Paradox tells her to call for him if she wants to spend time home. There they have one main adventure with the Founding Fathers of America that inadvertently creates the Plumbers in the Doctor's universe and Sati can pull her weight when needed from time to time. Other smaller adventures also happen as well before the third meeting with Rose happens and the rewrites begin.
Also, some rules I am going to play by:
1. Companions are going to enter and leave at the same time they would have without Sati, just for the fact that seeing how other companions will react to later episodes isn't something I am confident writing even with these little notes.
2. The Doctor will regenerate in the same episodes that they did in the show.
3. Episodes where I can't find a good reason for Sati to be included, like Midnight and Turn Left, will be almost entirely skipped with only a brief rundown on them.
4. Every companion will meet Rath. This is nonnegotiable.
I really hope this idea interests some of you and I would love to hear your thoughts and opinions. Also, for those curious, here is what Sati looks like(to view the image you have to right click and say open image in a new tab.)