I think that the assault style legs are needed, and will merge quite nicely with the existing sprues. The Judicar (apparently that's what the chaplain-like dude with the executioner sword is called) looks awesome in my opinion, and the bikes are simply awesome, even if they still have massive tyres and no ground clearance!
The Necron line is definitely in need of a refresh though, so I'm looking forward to what comes next. The Silent King is apparently bringing back some new toys! Amongst them possibly a new C'Tan? Thing with wings in the little picture.