Following the general site design guidelines will further improve accessibility.
Use a clear, consistent navigation structure.
Offer navigation bars for easy access to the navigation structure.
If your tool provides a "pre-production planner" that explains the need for designing a website to be user-friendly (accessible) it should include certain templates, especially a stylable "look and feel" to provide consistency of navigation methods within all documents created with the tool.
Provide a description of the general layout of the site, the access features used, and how to use them.
Offer a site map.
Whenever a site is produced by your Authoring Tool it should (unless trivially small) be organized by either author- or tool-generated site description and map. This will enable easier accessibility, particularly for first-time visitors. An easily (intuitively?) navigable site is a comfortable site.
Offer different types of searches for different skill levels and preferences.
If the author is to include a search facility it should be possible to use it without having a degree in database search techniques. It is important to remember that whenever someone must interact with the site produced by your tool that they don't need a lot of switching between pointer and data entry mode.
Ensure that nothing within the site prevents keyboard operation.
It is important that those who prefer to (or must) use the keyboard for navigation instead of a mouse are also customers of the Web site your customers are designing.
Ensure that text and background colors or patterns contrast well. (A good test is to print your page to a black and white printer).
Both the style sheet editor and the preview functions of your tool should have an "example box" so that the author will know what her users will be up against when attempting to view yellow on white, etc.
Test the site with at least:
a text-only browser (e.g., Lynx or a Lynx emulator such as Lynx Viewer or Lynx-me)
multiple graphic browsers, with:
sounds and graphics loaded,
graphics not loaded,
sounds not loaded,
no mouse
Validate pages with tools such as:
W3C HTML Validation Service http://validator.w3.org/
To expect an author to make all these sorts of tests is probably a bit much, but the inclusion of a means of viewing the site in a text-only mode will go a long way towards showing what a high percentage of potential site visitors will encounter when they are using accessibility aids or "hands-busy, eyes-busy" devices like palm-top or webphone browsing devices.