What’s funny is Disney is milking the Original Trilogy cow with Vader, Tarkin, young Leia, Yavin IV, etc. Now, they are using a young Han Solo, Chewbacca, Vader again, Lando, Jabba, Boba Fett, and the Falcon. Then, once they confirm it, they will be milking the Prequels with an older Obi-wan “Ben” Kenobi film, using Qui-gon Jinn, Yoda, Mace Windu and what ever characters they decide to use from the prequels or original trilogy, like Mos Eisley, the cantina, or a young Luke Skywalker.
Yes, Mace Windu will be in the new Obi-wan spin-off. We didn’t see him die, this will give him closure and to see him meet Obi-wan in the cantina in Mos Eisley talking about Anakin/Vader/Palpatine. They also go an adventure with the two of them as he sacrifices his life for Obi-wan at the end, so he can be there for young Luke.
Think about it, in The Force Awakens you have Finn, in the Han Solo film you have Donald Glover’s Lando. It makes sense to have Master Windu return with a bionic arm and bad ass attitude, he’s mad about what happened to him and the Jedi. He went into hiding, but Vader or The Empire never got him.
That’s how I would do it.