Things I've noticed are locked that could be useful to creating some nice designs:
Checkbox inputs - useful for "checkbox hacks" allowing pure CSS buttons to change styles on other elements.
The direct descendant css selector ( a > b )
The "tabindex" property entirely, which would allow CSS tweaks through the use of the :focus pseudo-element
The :target pseudo-element (not often used, but i've seen it on sites to make more pure css buttons, checkbox hack is a much nicer alternative)
The :before and :after pseudo-elements, understandably blocked but still extremely useful. Maybe look into limiting what they can be applied to, ex: only allow them to be applied to user-supplied elements (those in HTML input areas)
I'll probably be commenting with some more suggestions, these are just things I've found while trying to work out some accordion elements for some profiles.