While I'm becoming a fan of Gust, I'm incredibly unethused with the idea of a crossover RPG. It just seems like a recipe for storyless drivel. What I've heard about Chaos Wars does nothing to disaffirm that.
most crossover games have a story driven by orriginal characters. but judging by your post i'm guessing your the type thats like omg must have an epic l33t story! so i guess this game is more for people like me and less for people like you. i hope it has some ar tonelico characters.
edit: just wait a minute! didn't idea factory make dragon force? thats means they made at least 2 good games then :o