Why she might be dead: One of the biggest theories coming out of the finale has to do with TWD's use of the first-person perspective throughout the episode, and how its use might be a clue as to who dies at the end. Since the episode only features it three times — twice in the back of the van where Michonne, Daryl, and Glenn are being kept, and once at the end from whoever dies — it stands to reason that whoever was being kept in the van has a strong chance of being the one Negan kills. Michonne is in both spots, and her death would be a huge blow to the show's fandom (#beloved).
Why she might not be dead: Michonne is on Rick's right, so technically it could be her, BUT in the new footage, Negan makes a big deal out of insinuating that Rick has lost his "right-hand man." Despite Michonne being a woman, she can still be considered Rick's right-hand man given all the work she does, but we're going to hold on to this technicality with all of our strength in hopes that she lives.