Mara Jade

Stop insulting my literacy. It does not behoove you.

I use "Legends" in the sense of, "They are old canon, but not new canon." If you use "Legends" to mean, "They were in old canon," it becomes an almost useless term, because if that's the case, Luke is Legends.

So, by the useful definition of the term? Mara is Legends. Thrawn is not.

Now, as to the subject of Mara Jade in episode 9? That would have to be a very different Mara Jade, to the point of being a completely different character, risking much more fan backlash than if they introduced a new character inspired by Mara. Thrawn, on the other hand, is the same character, and was simply a refinement of what came before, and a case of Zahn mellowing out some of his excesses.

As to the notion of Mara being Rey's mother? That's the literal worst thing they can do with the character. That Rey's parents are nobodies is thematically key. Turning her into the daughter of magic superspy Mara Jade now would be worse than when Naruto turned around and made his bloodline the mostest specialist thing ever after volumes about overcoming despite coming from nothing.

Besides, it's obvious that Rey is the abandoned daughter of Lone and Vespa Starr.