Hi @eliranw,
you’re doing nothing wrong 
You will need to at least always add the form name to the domain file as the model has to know about which things it can execute in response to user utterance. I could imagine you could just define 1 placeholder form, and then dynamically switch the implementation during execution based on the conversation state (e.g. slots, last intent). However, you’d loose the ability to write stories / rules for this.
Another issue is that you currently need at least one slot mapping to be present in your domain.yml. So you can’t define a form without any slot mappings (only custom mappings). I created an issue as this is we want to solve soon.