We already have one. It has legislated nationally and finally on abortion, contraception, and now on sodomy. It will within a very few years legislate nationally and finally on marriage as well.
It consists of nine unelected legislators-for-life, six of which are drunk with legislative power and desire to impose their own personal sense of morality, of a strongly liberal cant, on the other 260 million Americans.
We cannot effectively impeach them; we cannot appeal the legislation they impose. They are all-powerful.
No they aren't.
They are entirely subject to the limits Congress places on their appellate powers (see ex parte McCardle).
If there are enough votes in Congress to amend the effin' Constitution, there are certainly enough votes to limit the jurisdiction of the Court over this and many other matters.