I think maybe Canon has shown their full hand for NAB already, announcing the C300 Mark II and XC10 (I'll be discussing those in a post very soon). I really don't think we'll see a C500 update at the show. It just doesn't make sense to wait a few days after announcing the C300 Mk II; why not just announce them together?
With the FS7 having just started shipping recently and the relative newness of the a7s, I doubt Sony will have any earth-shattering announcements.
While we won't likely see any new products (at least in the consumer/enthusiast range), I think the major GH4 firmware update rumored to include external RAW recording is a strong possibility. Introducing something like this for the GH4 mid-product cycle, is brilliant marketing. I really don't think RAW would squeeze that much more out of the GH4's sensor as compared to what's already possible with 4K 10-bit 4:2:2 ProRes with the Shogun, so this likely wouldn't cannibalize any higher end Panasonic offerings. The camera's fairly poor low light performance and mediocre dynamic range are really the limiting factors in its image quality and RAW probably wouldn't do a whole lot to remedy these issues, but It would create a low of buzz, as this would be the only camera under $10K (even with the needed accessories for RAW recording) other than the Blackmagic Production Camera to shoot 4K RAW. Perhaps the update would also include a true LOG picture profile, which may help the DR a bit.
I think Blackmagic really has the opportunity to steal the show. It's been two years since we've seen a new sensor in their cameras, and with the new URSA firmware update, it seems almost certain that they will be introducing a new sensor. A new 4K camera with better ergonomics, better lowlight performance, and slightly better dynamic range could prove to be another game changer. I also wouldn't count out the possibility of a 6K camera.
DaVinci Resolve 12 is a certainty, with new audio finishing features. Could this mean the new Blackmagic Cameras will finally have proper XLR inputs with phantom power and better control over levels to go along with this new focus on audio?