What was The Thousand-Year Door's tentative North American title?

The Thousand-Year Door's tentative North American title was Paper Mario 2.

Before its final international name was settled, Nintendo referred to the GameCube sequel as Paper Mario 2 in North America. The title clearly signaled its relationship to the original Nintendo 64 Paper Mario, which the new game followed directly. In Japan, the project was tentatively known as Mario Story 2, reflecting the Japanese naming tradition for the series.

The finished game became Paper Mario: The Thousand-Year Door, a title drawn from the ancient door at the center of its mystery. It was released in Japan in July 2004 and in North America in October 2004. Players sometimes remember “Paper Mario 2” as an alternate release title because it appeared in previews and early promotional material, but it was not the final North American title. The game remains the second numbered mainline entry in the Paper Mario series, despite adopting a more descriptive subtitle.