Well.. FFTa is essentially a big ole' compilation of FF references.
I figure FF12 is a more serious approach to the.. 'original universe' of Ivalice? (FFTa's Ivalice being some sort of 'alternate universe')

I don't think that FF12 and FFT are going to be related, or perhaps by some odd chance that they might be a part of the same universe at different time periods.
We'll have to see.