Yeah, I doubt Square-Enix'll be making any sort sequel for FFVIII.
1 - Because of the mixed opinions of FFVIII, it wouldn't make as much money as Advent Children will, seeing as how FFVII is usually the people's favorite FF.
2 - Like Abominatrix said, the ending closed off any possible continuations of the story. To make a sequel would be ridiculous. Absolutely ridiculous.