Make steps more configurable when populating existing values in form from previous step

This is a follow-up for: Divide the embed form in two steps

Relevant code:

   // Populate existing values, i.e. embed_method, entity_type and entity.
   var existingValues = {};
   existingValues['entity-type'] = response.values.entity_type;
   existingValues['embed-method'] = response.values.embed_method;
   existingValues['entity'] = response.values.entity;
   existingValues['editor-id'] = editor.name;

As @slashrsm suggests:

Think a little about how to make this steps configurable. Let's say that I want to completely swap out this step and replace it with something else (or even remove it entirely and just use some defaults). That should be possible and shouldn't allow much hacking with alters and stuff. Could we utilize plugins to achieve that?