I feel that setting the major party threshold at 6 members, one for each subregion, would be a good compromise for the conundrum of minimizing zombie parties while keeping the barrier to formation of new parties relatively low.
A BILL FOR AN ACT
Establishing a legal threshold for federal party status
BE IT ENACTED BY THE SENATE RESOLVED.
Section 1, TITLE.
i. The title of this Act shall be, the “Proportional Party Majorness Agreeable Gateway Act.”
ii. This Act shall not be referred to using the title's acronym.
Section 2, THRESHOLD.
i. A federal political party shall be considered a "major party" when the party in question obtains a membership that consists of at least 3 6 registered members.
Section 3, ENACTMENT.
i. This bill shall take effect starting with the first census held after either signature by the President or a veto override.