Why's that? (It is a pre-established design and, IMHO, makes the most sense if you wanted a saberstaff; more compact and easer to cary when not in use.)
I think the dark side Rey thing is about the same thing as her seeming to be joining Kylo at the end of the second TLJ trailer; a shocking bit that's taken out of context to get people talking. I seriously doubt she's actually going to go bad for real (maybe a Force vision or an illusion of some kind?). As far as the trailer itself, it's so vague that there's no real clue what they story is about (beyond the obvious "Resistance vs.First Order" that we all knew was coming). Way too early to be judging it, IMHO.
Actually, the crossgaurd is purely for function; since Kylo has a cracked Cyber crystal in his saber (hence why the blade crackles and looks less solid), he needs it to vent the excess energy to keep the saber from overloading. (That said, the crossguard design does exist for its own sake.)