Jedi Master Mace Windu event-Is it ever coming back?

No. If you didn't unlock JMMW already because a lot of people were saying he sucks and isn't worth the effort, you missed out. He's never coming back. Legendaries and GLs will now have 2 appearances: one in Era, one out of Era. If you don't get them during either of those events, you never will. It's the new exclusivity model. Did you miss the forum post announcing this update?