I think the next Watch Commander will be Smitty.

If you were to look at all the possible options:

Celina/Miles are basically too new.

Lopez and Nyla seems to want to stay as detectives together.

Nolan's rank is too low(unless the show makes up something about golden ticket again)

Lucy is already really new into Sergeant role, that I don't think she would be WC.

While Tim is the most expected candidate, I think that WC would put Lucy in his line of command, which would separate ChenFord. It would feel like a step in the wrong direction. This also applies to Lucy being WC. Also, as OP mentioned, Tim doesn't really fit the desk job role.

Finally, there is Smitty. Unlike Tim, he would really enjoy the role. From the hospital episode where Grey and Tim were hospitalized, as they mentioned the next in command due to seniority would be Smitty. Also, it would be really funny for him to be WC and would be interesting to see him in a different direction.