What I've seen of Athos is great, and you work with Porthos and Aramis really well. Their conversations feel very natural and they come across as old friends who're comfortable around each other.
I would love to see Athos being developed individually as well, especially because of his canon point. How is he dealing with the aftermath of the encounter with Milady? How is his alcoholism now, has it changed to how it was before, now that he should be in a better place emotionally? More specific to the game itself, does he have any thoughts regarding the different time periods he could learn about while being in Miracle Country, particularly modern ones? How has he been adjusting to living in a modern setting with no musketeers or anything of the sort being needed?
He's a very interesting character, I would love to see things like that and more being addressed.