Sony hasn't announced whether there are any other Godzilla DVDs they plan on releasing along with it, like they did with Tokyo SOS. It' possible that they don't have the rights to any other Godzilla movies at this point, but personally, I think the following might be likely in the future...
Godzilla Raids Again (they got Smog Monster, which was OOP for a while, I got my fingers crossed)
Ghidrah, the Three-Headed Monster (was often released with several other Godzilla movies that Sony has already got out, might be likely)
Godzilla vs. Megalon (ditto)
Godzilla 1985 (ditto, although a Japanese track would be difficult, since it was heavily re-edited complete with scenes involving American actors)
Godzilla vs. King Ghidorah (a lacking original DVD from Sony, maybe it's time for an upgrade)
Godzilla and Mothra: The Battle For Earth (ditto)
Godzilla vs. SpaceGodzilla (ditto)
Godzilla vs. Destroyah (ditto)
Godzilla 2000 (although it's DVD is actually pretty damn good, it's lacking that beloved Japanese audio track)
The following can probably be crossed off the list...
Godzilla, King of the Monsters (Classic Media releasing it)
King Kong vs. Godzilla (Universal's releasing it)
Godzilla vs. Mothra (Classic Media again)
Godzilla vs. Monster Zero (ditto)
Destroy All Monsters (ADV has it)
Godzilla's Revenge (Classic Media again)
Terror of Mechagodzilla (ditto)
Godzilla vs. Biollante (HBO Films used to have it, they might still do. I'm not sure)