attr() on all properties and on regular elements would be great.
There was some great ideas in the 2003 Generated and Replaced Content draft, like ::outside pseudo element, ::before::before::before etc pseudo elements, deferring rendering with an ::alternate pseudo element. Wish we had those.
If we can't have 2, then content:contents would be very helpful.
Oops, that's five features. Just being greedy :-)
Getting attr() on all properties would be absolutely great! If I had to pick one of these, it'd be most definitely that one! :)