# test_role defaults/main.yml

test_role_var = 'something'
test_role_var_two = "{{ test_role_var }} blah"

# test_role tasks/main.yml

- name: Some task
  task:
    task_var: "{{ test_role_var_two }}"

# test_role2 tasks/main.yml
- name: Include test_role
  include_role:
    name: test_role

# On the CLI:
ansible-playbook --extra-vars 'test_role_var=something_new' playbook/use_test_role2.yml