Yeah, i'm a fan, it's one of my favorite since i'm a big fan of fighting games(specially the 2-d kind). Zappa is defenetly my favorite character from the GG series followed by Potemkin, Faust and A.B.A. I like Zappa's unpredictebility, and summoning the dog just to annoy is one of my guilty little pleasures .
I havent heard about any GG sequel that will advance the story so far, but there are upcomming remakes like Guilty Gear XX Slash, it will have asome new moves, a more balance A.B.A. and a new version of Sol, here's some pics i've found so far:
The New Sol
And there's also that new Sammy vs. Capcom game, but i havent heard of much details.