I wouldn't say the book on Mara Jade is completely closed, but I will say, she would't fit into Rebels . Mara was an extremelt popular character. In fact, she was the only EU character to ever make the Top 20 list of all time favorite SW characters. Even Thrawn didn't make that list.If you're going to introduce a popular Legends character, Thrawn is a very safe choice. I liked Mara Jade and I like the idea of Emperor's Hand characters but the window for introducing her is closed. That said, I approve and 13 year-old me from 1991 approves.
*blargh!*
Even in the original EU, Thrawn's "exile" story was a cover for an actual mission Thrawn was assigned in the Unknown regions. Namely as a vanguard against the Yuuzhan Vong.
The Yuuzhan Vong fall so far outside of the central narrative theme of a fairy-tale space fantasy that I'm still baffled that, as a concept, they made it past Lucasfilm's book group. Jaxxon the Space Rabbit was awful as well but he (sort of) fit the milieu.
Imagine that, a bunch of Star Wars nerds voted for the stunning red head in tight pleather as one of their favourite characters..
[yoda]and wrong with that, what is, hmmm?[/yoda]