Of all of the features in Sizzle, positional selectors (:first, :last, :eq, etc.) are the most cumbersome and problematic. If these were removed from Sizzle, it would greatly simplify the codebase, reduce code size drastically, and set Sizzle on the same track as the Selectors spec. Obviously, this would have a major impact on user code, but we could start with a branch and make it an optional replacement in jQuery. Hopefully, we could see positional selectors phase out of Sizzle completely some time in the distant future.

Note: this is a much more significant size decrease than dropping old IE.