How to Create a website BuilderCreating a Website Builder
Already it has just about all the functions you want. Have you ever listened to Ucraft's white label website software? That' exactly what you are looking for when you want to create a Website Builder. They have two price plans: default and self-hosted platforms. With the first one you can create a website builder like Ucraft: with all its integration like Google Analytics, Mailchimp, etc. and product like Logos Make, Hosting Page Builder and more.
This self hosting site gives you everything you need above, as well as accessing the sources: this means you can modify and append anything you like. Things you describe already exist with Webflow and it's better than you could ever make yourself. As well as creating HTML for you, the drag-and-drop editor also creates the HTML for you, as well as the CSS and JS.
Best of all for me, you don't need any template, plugin or extension, so creating customized pages is actually a lot simpler. The addition of user-defined codes is child's play and the data is only a small part of the file sizes of a similar Wix or WP site. Yes, it is indeed possible to create a website builder that supports many CMS with HTML5 codes.
I' ve already used a website builder with more functions than you list here, but it works off-line when you are installing it. TemplateToaster is the name of the program. It is very versatile and fully compatible with all common CMS like Magento, Joomla, Drupal, Wordpress and more. You do not even have to create a unique piece of coding on both the frontend and the backend with this Drag and Drops Builder.
SP Page Builder for Joomla or Divi for WordPress.