---
  - name: create database
    mysql_db: name={{ mysql_user }}  login_password={{ mysql_root_pwd }} state=present

- hosts: somehost

  tasks:
  - include: roles/rootserver/tasks/mysqluser.yml
      vars:
        mysql_user: "{{ item.name }}"
      with items: "{{ typo3 }}"

mysql_root_pwd: sdhjouidhjuihe3
typo3:
- name: foo

The offending line appears to be:

vars:
   mysql_user: "{{ item.name }}"
   ^ here
We could be wrong, but this one looks like it might be an issue with
missing quotes.  Always quote template expression brackets when they
start a value.