How do I display all available field types? I want generate an new form in a custom module via the generate:form command.

In this step, I must enter a field type. 



How can I get all available field types?
With the term "field type" you probably don't find what you are looking for. Better search for "form and render elements". You can find a list of all available elements here:

https://api.drupal.org/api/drupal/elements/8.2.x

You can use all elements of the type "FormElement".