There's a big discussion that I opened many years ago about FFXI's plot holes etc. One of the things discussed was exactely that. What are "the gates of paradise", behind all the metaphors it gets used with?
Opening the gates of paradise shouldn't be seen as a literal portal to another dimension, the "real" vana'diel.
It's more a metaphor for a process that would merge together once again the 5 crystals into the mothercrystal.
Doing that would destroy vana'diel (and the universe?) and all the races of vana'diel etc. The final result would be the "world" (we can't really talk about a "world" here, but I can't find a more fitting word) where Altana and Promathia used to live.
The Zilartians wanted to achieve that, live together with her goddess in that "world".
I seem to recall it was something like that at least...