16th level characters shouldn't struggle with the Tomb of Horrors.
You ought to have enough character history that the story from here is driven by what the characters want. They're powerful, rich and successful. Ask yourself (and if that doesn't help, ask the players) questions like: Who used to kick sand in their eyes? Are any of their countrymen or co-religionists oppressed? Is there a dangerous frontier that should be secured, or a degenerate empire that needs a new monarch?