From the details that have come out:
Rules are staying the same as now, but there will be optional rules added, such as variant balls (squig!), and there will be optional rules to add a more narrative feel to seasons, rather than just being all about winning.
Pitch is the same in number of squares, but is bigger. Each square is 32mm (up from approx 29mm) so adds about 7-8cm to the length of the board, meaning it won't fit our tables. However, GW are aware this is a problem for many people/clubs, hence not changing the rules.
Range ruler has been scaled up to fit new pitch.
Pitch is quad-folded and double-sided - orc pitch on one side, human on the other. Pitches for other races
might follow.
Looks like Pact and Underworld will be in the rule book. Slann probably not, but might be added later, but they not outlawing them.
And if it sells well, they're looking at irregular supplements that will concentrate on different types of play, such as in the Chaos Wastes (looks like someone read our ARBBL Goes Chaos rules
).