Godzilla 54 is currently well in the lead, and I can definitely say I'd be interested in purchasing that figure. In addition to being the very first Godzilla design, it has a very "Charles R Knight" look, giving it a charming retro dinosaur feel. It would also be another Showa character, which this line needs more of.
It's a bit funny that we all have such fond memories of Super Godzilla for the SNES, given that the game itself could at times be very frustrating. Even so, Super Godzilla is currently #2, and while I imagine that many would prefer to see other Godzilla designs first, there is no denying that Super Godzilla is rather distinct. I'd be interested in this character as well.
In third place is Titanosaurus, a figure I would wholeheartedly approve of and one I'd certainly purchase. Titanosaurus has been requested for quite some time now, and since Biollante was made after fan feedback I wouldn't be surprised if Titanosaurus follows suit.
As for Bagan (who certainly gets talked about a lot in this thread), there are at least three distinct versions of the character that can claim the title of 'official design'; the suit version, the final concept version, and the Super Godzilla video game version. For a quick look at each, you can check out the awesome sculpts by the very talented AWMGodzilla62:http://skullbrain.org/bb/viewtopic.php?f=49&t=43000
If the design is considered, I'm honestly not sure which version Tamashii would go with. The video game version probably has the strongest claim, but I think my favourite is the suit version.
But Bagan still is a pretty long shot, all things considered. If Super Godzilla is made, he should totally need to have his beam though; a massive clear plastic explosion with Godzilla's face at the end = awesome.