The Billie Jean figure IS the first Michael Jackson that HT produced. A lot of people didn't like the prototype sculpt, and the finalized HS was changed a few weeks before release. Not a lot people ordered it because of the proto, hence a low production number was made. Then MJ died, and everyone was fighting for one.
The Godfather figure, the tsunami version, from what I remember was a very small batch re-release with each figure hand painted by J.C. Hong with certificate number. That one is rare a should fetch for a high price.
Another rare figure (probably drawing no interest from western audiences), was Wong Ka Kui the leader of Hong Kong band "Beyond". He died young due to a accident while performing in Japan. Probably close to Kurt Cobain in terms of Hong Kong legendary pop icon status. HT made 2 different figures of him, one with a green jacket variant limited to 40 pcs.
I'm sure there are plenty of other rare HT figures for you to do a follow up, like the battle damaged endoskeleton and T-600 concept version.