Guest characters eh. Well I ranted about this on my FB, lol, but I was reading the SC wiki (I honestly have no idea how valid that source is) and saw this under trivia:
"Daishi Odashima mentioned on his twitter that he would like to add Final Fantasy characters as guest fighters, therefore hinting a possibility for the first time that Final Fantasy characters appears in the upcoming game."
Now as I said, I have no idea how valid the SC wiki is and I personally would prefer there be NO guest characters, but lets face it. Namco is a business company and they will do what it takes to make money and get a larger audience for the better (or what they deem as better). Add on the fact that Square Enix has made a lot of dumb business decisions as of late (Pissing fans off porting FFXIII to 360 in JPN. FFXIII-2? Where the fuck is my Versus XIII) and aren't on the best financial terms right now, they could probably use the money if Namco were to shoot them some for the license to use some of their characters. So, the idea of FF characters appearing in SCV doesn't seem so far-fetched to me.
Now if it had to come to FF cameos in SCV, I'd accept any of the cast members who AREN'T from FFVII, FFVIII, and FFXIII. What I would PREFER though, if it had to come to FF cameos, would be any character(s) from FFT or FFXII (aside from Vaan because I hate Vaan). The world of Ivalice compliments the setting of SC nicely and logically.