base.yml

- hosts: all
  connection: local
  tasks:
  - include: otherfile.yml
    vars:
      aa: "aa/{{item}}"
    with_items:
      - A
      - B

otherfile.yml

- debug:
    msg:
      a: "{{aa}}"
      it: "{{item}}"
  with_items:
    - I
    - J

output.log

PLAY ***************************************************************************

TASK [setup] *******************************************************************
ok: [localhost]

TASK [include] *****************************************************************
included: /home/dustin/tmp/nest-test/otherfile.yml for localhost
included: /home/dustin/tmp/nest-test/otherfile.yml for localhost

TASK [debug] *******************************************************************
ok: [localhost] => (item=I) => {
    "item": "I", 
    "msg": {
        "a": "aa/I", 
        "it": "I"
    }
}
ok: [localhost] => (item=J) => {
    "item": "J", 
    "msg": {
        "a": "aa/J", 
        "it": "J"
    }
}

TASK [debug] *******************************************************************
ok: [localhost] => (item=I) => {
    "item": "I", 
    "msg": {
        "a": "aa/I", 
        "it": "I"
    }
}
ok: [localhost] => (item=J) => {
    "item": "J", 
    "msg": {
        "a": "aa/J", 
        "it": "J"
    }
}