I haven't read the Necron codex, but I think that perhaps that one and GK WERE designed with 6e in mind.
Thinking on that, here's things I think we might see:
1) Different types of Power Weapons giving different bonuses (Spears +1I, Axes +1S, Sword 6+ Inv save, Hammers as now, etc).
2) Rending on 4+
3) DECENT Rapid Fire Weapons
4) Weakened Cover and Transports
5) Force Weapons no longer cause Instant Death
PS. Has anyone ever thought that using the Fantasy rules for determining cover would be pretty damn good for 40k?
PPS. Anyone want to see Kill-Team and Combat Patrol rules in the book? Sort of a "get started in the game" set of rules?