@zenzuki
I am personally against this idea entirely. There are two main issues with this approach Visually and Mechanically.
Below I have added a photo of several Sea of Thieves characters and a photo of the new and improved Xbox Avatars to be added next year for comparison.
Visually:
The Xbox Avatars are way more simplistic in creation vs the Sea of Thieves counter parts. While the two character model systems have a few similarities the differences are too important to overlook. To simply to copy and paste the Xbox Avatars into a game not built for them is irresponsible as it would ruin immersion.
If you notice Xbox avatars are very rounded in their appearances/features/structure and do not have defining features. While the Sea of Thieves characters have a lot more edges and sharp aspects to their designs and features. SoT has detailed skin shading while the Avatars do not. There are a lot of small details that Sea of Thieves has and while one of two small details may have different levels of importance. It is the small details that make up the whole portrait.
Also as you will see that there will be a very wide assortment of character options to differentiate our pirates.
Mechanically:
Furthermore you then run into the issue on height. As we are all aware, Xbox Avatars allow you to significantly adjust your height. All released Sea of Thieves content has shown character models at roughly the same height.
This is on purpose as a smaller character is harder to hit and will cause an unbalancing in the game mechanics.
Rare would have to build an entire layer to the game mechanics to compensate and balance this issue.
So visually AND mechanically Adding the Xbox Avatars would cause problems in the game.
The other problems of what if they did add the Xbox Avatars:
The basic process of adding in the Xbox avatars as is a lot more complex than you might think at first. Then once it was added there would have to be remodeling/stabilization efforts for the weapons, equipment, and everything else due to the new forms of character models. Then a game wide bug testing for these new model types. There are a whole plethora of problems to be had that cost more time and money. All of that plus the whole height issue.
Last Thoughts:
So I think that if you want even more character customization then as a community we need to express our desire for more options to be added Post-Launch. As them adding new stuff in a character system already made for the game will cost significantly less time and money.
However on the flip side of your idea I would love to see our Sea of Thieves characters be allowed to optionally replace our Xbox Avatars.