I think if Legendary decides to make more Godzilla movies, Biollante could be a perfect choice for them. With modern special effects and CGI, they could do some really nce work with a monster like Biollante. The original movie was hampered by it's budget and SFX of the time, but with some serious work behind the old gal, imagine it, vines crawling up skyscrapers, crowds running from the teeth-filled limbs, Biollante could look real good with modern SFX.
Also, it'd make a good "second enemy" for Godzilla, first he has to be established, second he must meet a challange as great as himself, third he must overcome something greater (Destroyah-SpaceGodzilla-King Ghidorah), then the legend is complete, and they can work from there.
They wouldn't necessarily have to use the same origin for Biollante. It could be created by Godzilla's blood being absorbed by some plant life. Or like the original, some scientist, seeing Godzilla's endurance tries to make a stronger plant, playing God ensues, and arrives giant plant monster. The having a human soul aspect isn't necessary, a standard "mutant" type story would do. To be honest, having the girl's soul wasn't really necessary in the first movie either.