Javascript already has the ability to define new abstractions. They're called functions.
Imagine you want a new "tag" for defining a Widget. Make yourself a Widget function which takes some arguments, maybe the name of the widget, the mass, and the price. Those arguments can themselves be HTML, created either using E4X or any other means you like. This lets you compose things together. For example, perhaps the price could just be a number today, but tomorrow you want to let the user do currency conversions and so you make it a more complicated AJAX thing that lets the user view the price in any currency they want. Today you would have Widget("Plumbus", kg(1.1), 49.99), but tomorrow you could change it to Widget("Plumbus", kg(1.1), Currency(USD, 49.99)). No new inventions required.
Sure, but the why are we talking about special syntax in the first place? In any non-trivial projects you're rarely going to be working with primitive tags, it's always higher level components.
Because all higher-level components must be created out of primitive html tags with a sprinkling of attributes, and it's nice to have a convenient way to create those. This is the same reason why modern languages include some kind of object-literal syntax rather than relying purely on constructor functions. Even C has a comprehensive syntax for initializing objects now. E4X was just that for HTML and XML.
Imagine you want a new "tag" for defining a Widget. Make yourself a Widget function which takes some arguments, maybe the name of the widget, the mass, and the price. Those arguments can themselves be HTML, created either using E4X or any other means you like. This lets you compose things together. For example, perhaps the price could just be a number today, but tomorrow you want to let the user do currency conversions and so you make it a more complicated AJAX thing that lets the user view the price in any currency they want. Today you would have Widget("Plumbus", kg(1.1), 49.99), but tomorrow you could change it to Widget("Plumbus", kg(1.1), Currency(USD, 49.99)). No new inventions required.