There already is a sort of visual successor to Greasemonkey: Stylish, at http://userstyles.org/. As Greasemonkey lets you write custom JavaScript, Stylish lets you write custom CSS. But it’s not exactly a tool for the masses, since you have to write CSS by hand. And you’re lucky if the site you’re styling has a clean enough DOM that plain CSS is enough – I drop to writing JS-enhanced CSS in Greasemonkey when the site has that problem.