Snippets from the interview (unconfirmed): GX's story is inseparable from G, if G could be considered the "front" then GX is the "back." In other words, 2 parts of the same whole. Hibiki is the focus of the story, the weight of the plot will fall upon her. There are no new ally characters who will be come gear users, however there are new characters that appear who are a fundamental part of the story. Further, a strong new enemy will appear that, unlike in G, is capable keeping the Symphogear's power in check.
Finally, there have been some rumors flying around about a new Symphogear-themed panchinko/slot machine that will be available this summer along with Marcoss F and Infinite Stratos...BUT these are only rumors flying around and I have yet to see anything concrete to confirm it. We should know for sure soon.