As you all know, there was a great demand for a Final Fantasy character to make it in the roster for this game. We know that dlc is coming, and it has been implied that there could be more beyond Kat and Emmett. Could that dlc be a Final Fantasy/Metal Gear 25th anniversary celebration pack? If so, could Zack be the Final Fantasy character? Let's look at a few potential hints.
http://www.animenewsnetwork.com/interest/2012-12-18/final-fantasy-and-metal-gear-bento-lightning-returns-design-revealed-on-25th-anniversaries As you can see, Sony did special bento boxes for both franchises. Interestingly, out of all the potential Final Fantasy games to base it on, Crisis Core was chosen as the bento box. What's of more interest in this story, though, is that Sony will apparently be announcing new 25th anniversary projects for FF & MG on Wednesday. Lightning Returns, for those that don't know, is multiplatform. While it's possible that Sony will be announcing new details on that game, it seems more likely to be announcing a Sony exclusive. If the announcements are for both franchises, couldn't it be one game that could combine the two? Couldn't that be PSABR? Snake and a Final Fantasy character are highly requested for the game, and it seems dlc characters will be released in pairs of rivals. Perhaps Superbot & Sony will put out a dlc pack that features both of these beloved franchises.
Let's also take a look at Zack Fair specifically. He's popular. Really popular. Way more popular than many Final Fantasy fans realize. http://www.novacrystallis.com/?p=3887 According to a recent poll on the official Final Fantasy forum in Japan, the fans there like Zack the most out of all male Final Fantasy characters. With that particular group of fans, at least, he's more popular than longtime favorite Cloud Strife. It's also worth noting that Zack is the star of a very successful PSP title, and PSABR doesn't really have a playable character that primarily represents that system. There are characters that had PSP games on the roster, but they're more known for their PS1/PS2/PS3 titles. Vita is getting a rep by way of dlc, so why not PSP? Crisis Core is also a Sony exclusive title.
Could we be getting FF & MG character announcements soon? Maybe. My preference would be Cloud, but I could definitely see Zack getting the nod instead. As for MG, I'm sure the choice would be Snake. It's really just a matter of which Snake.
Technically, all Snakes can be represented through different skins. Just a matter of personal preference to the person playing them. Same as with Cloud and Zack, they would have relatively the same moveset, so a skin swap would be all that is needed to satisfy most people.