I'd like to move to Drupal 8 for my next project. I understand that, from version 8, Drupal standard template engine is Twig.
Is it possible to build a brand new Drupal 8 theme without using Twig at all?
Consider using React, Angular, Vue, or Gatsby as an application development framework with Node.js & use GraphQL for the Drupal 8 integration with other APIs, i.e. a decoupled or headless Drupal CMS solution. You may still need to use a Twig theme for the D8 administration section, although there are also projects in development for that.