^^^
I believe that too. I remember someone mentioning that the "fire dragon" was actually a ship from an older TOHO or Godzilla movie, like "Destroy All Monsters"...
Whoever reported that though, was not exactly thorough....More like last-minute reporting to me.
On Topic:
The Con Air storyline would've SUCKED. Think about it: What would convicts do against godzilla, besides attempt to bomb him or use the Yakuza to find something to do to him? It would've made no sense and had no purpose in being a "final" film.
The Minilla death storyline, would've made sense because with Minilla terminated, Godzilla's death would mean a lot, being that there'd be no one to carry on the legacy and that would've been his last shot. Having him die slowly in front of an audience, like in GvsD, would've made a greater dramatic impact and brought out that extra thematic element to make the movie memorable.
However, the idea Final Wars ran with is, in retrospect, sort of the best one because it's a nod to Godzilla's past and TOHO's legacy with kaiju/sci-fi movies.