What would be the next series letter after AH-1Z?

According to the most recent edition of DAFI 16-401 (bottom of page 20):

A2.1.7.3. At the end of the series symbol “Z”, the next sequence will be to advance the design number to the next consecutive unused number and begin with series symbol “A.”

So, for the specific example of AH-1Z, assuming this somehow were to happen right now, the next version should be designated AH-74A (since "74" is currently the next available Design Number in the Helicopter series).

The very next sentence is also interesting: "A2.1.6.1.2. AF/A8PE reserves the authority to skip design number at discretion." Thirteen is a number that has been skipped a lot, and another one that was skipped recently in the Helicopter series is 69.

Also interesting to note "...Design numbers run consecutively from “1” to “999”... and "In the event that a design number of an aerospace vehicle reaches “999”, the design number range will be expanded to “9999” and future design numbers will
be assigned accordingly." I don't think that will happen anytime soon. The highest standard Design number is probably 184 in the missile series (RGM-184).