E.
The three letters used for different models of Xbox are X, S, and E. We haven't seen an E since the 360. There is a reason why they wouldn't want to use it due to what S, X and E could spell out but it's one they've used before.
Z is a possibility, maybe if it's a clamshell or has a kickstand?
I think they have lots of flexibility with the Series brand. Maybe we see a Series XZ for the Pro model.
Of all letters of the alphabet, I think the candidates would be:
E, G, H, I, K, M, Q, R, (obviously S), T, V, (Again, obviously X), and Z.
All the rest have implications or are used for something related. U as in Wii U, C as in Type C, A and B as in A-B testing, F as in failure, D as in unmentionables, same reason for P, J as in joke or joker, L as in loser, N as in no or a slur, O as in zero, Ordinary, or a passing grade that just barely passes, W because it has too many syllables, Y as in why.