Except in many cases you can't make layout decisions without also considering fonts and visual style.

This is why Photoshop and Illustrator are still the best tools for web design: they enable relatively quick wireframing, but also give you the possibility of increasing the level of polish within the same document.

If a new alternative is to appear, it will need to provide at least support for realistic grids, typography, and basic CSS styles, not just empty boxes and Comic Sans.