I have to praise PowerShell here. You get tab completion [0], type checking, automatically generated syntax help text etc. for free, just by writing your CLI tool in PowerShell.
Then you can also use a simple, built-in, declarative DSL to gradually enhance the robustness of your CLI by adding more checks or constraints, or use "parameter sets" [1] to define which parameters can and cannot be used together — tab completion will behave accordingly and suggest only what's allowed.
Dynamic completers, like mentioned in the article, can also be done [2].