Unfortunately, the thing I would like most is a complete replacement for LaTeX that offers these features:
* Text-based - no binary file formats
* A unique DSL for all features
* Complete control over layout and formatting
* Complete control over typography (alignment, leading, kerning, spacing, document grid alignment, etc.)
* Support for tables, including captions, headers, styling, and the ability to span multiple pages
* Support for images and image formatting (resizing, borders, alignment, margins, etc.)
* Reusable libraries/packages
* A package management system (de-emphasis on this one because good package management systems are... difficult to make)