How do I dynamically take variables from previous plays in Ansible?
It feels dirty, but I guess something like that would work:

- name: ec21
  [...]
  register: ec21_result

- set_fact: end_result= "{{ ec21_result }}"
  when: ec21_result|succeeded

- name: ec22
  [...]
  register: ec22_result
  when: ec21_result|failed

- set_fact: end_result= "{{ ec22_result }}"
  when: ec22_result|succeeded

- name: ec23
  [...]
  register: ec23_result
  when: ec22_result|failed

- set_fact: end_result= "{{ ec23_result }}"
  when: ec23_result|succeeded

- name: Add new instance to host group
  add_host:
    hostname: "{{ item.private_ip }}"
    groupname: launched
  with_items: "{{ end_result.instances }}"


Basically setting the end_result as soon as you succeed a play.