Actually, 4 and 6 were released in the US as 2 and 3, because the actual 2, 3, and 5 were never released outside Japan, and they wanted to avoid consistency problems.

And I know that someone is working on a FF6 prequel - supposed to take place during the War of the Magi. But it's an independent project (a team of people with no affiliation with Square), and it uses the same engine as the original FF6. I'm not sure if they're still working on it, if they've given up, or who it is that's doing this.