Oh, and another thing not to do:
Don't repurpose a command-line flag intended for one purpose for another unrelated purpose.
For example, don't repurpose a command-line flag (for concreteness, let's pick the hypothetical example of a flag called "--extcap-version"), intended for one purpose (in this hypothetical example, used to ask a program to report its version number), for a separate purpose (in this hypothetical example, used to tell a program the version number of the program running that other program), with the two unrelated meanings distinguished based on whether the flag has an argument or not.
I.e., for Wireshark 3.0, let's do the latter function with --extcap-wireshark-version X.Y or --extcap-wireshark-version=X.Y, with --extcap-version *never* taking an argument and *always* meaning "report your version number" and with --extcap-wireshark-version *always* taking an argument and *always* meaning "here's the version of Wireshark that's running you".