A game with Seifer as the main character sounds fun, but you'd lose the part of the game that is so rewarding a proper ending.

Sure, sad endings work, but Seifer didn't accomplish anything. He fulfilled his dream, but it'd just end after Lunatic Pandora. He disappeared in time compression and then when it was over he was fishing. It offered enough closure in FFVIII because he wasn't a playable character.

The ride there would be fun, sure. For a long time, you wouldn't have any other playable characters except his lackeys every so often, but then they'd go off to fight Squall & co.

There's too many holes to fill.

Now a game with Laguna, Kiros and Ward would be fun. You'd even be able to start before their antics began in FFVIII, like when they met each other. When they split up you could follow them as well and you'd get a look into Kiros and Ward.

Now that's a game I'd play.