Based on the recent rumors (which I am assuming are accurate, based on their level of detail and seeing them in multiple places), it looks like the Avatar of Khaine gains some improved abilities.
Now, given that all this is true, combining these two means you could potentially run an effective “foot-mobile” close combat battle group. Imagine the following:
-3 x units of 10 Banshees, each with Exarch and Executioner upgrades (from Aspect Host formation)
-Farseer with Bike, Mantle of the Laughing God, and Telepathy powers (Shrouded is best, since you can use it to protect all of the units within 6″ and even give them a 3++ cover save when they are in terrain).
Have all the units move up field as quickly as possible, using Fleet, Battle Focus, and Banshee extra Run bonus. Once in range, charge your selected unit with Banshees. As long as the Banshees are in range of the Avatar, each squad should get 36 x S4 AP3 attacks at I5 (and ignoring overwatch, due to new version of Banshee Masks) and 4 x S6 AP2 attacks at I6 (from the Exarch with Executioner). On top of this, if Banshees are taken from the Aspect Host formation, then they can choose to have +1WS, so WS5, meaning they are hitting most units on a 3+. Against MEQs, that translates into 24 hits and 12 AP3 wounds, so 12 dead Marines, not counting the Exarch’s WS6 I6 S6 AP2 attacks. This also doesn’t even count the Avatar himself getting into combat and wrecking house with his MC abilities and S8 AP2 Wailing Doom attacks at initiative.
As you can see that is a very potent unit that is a real threat in competitive games, especially compared to the old version of the Banshees. Combine this with multiple redundant threats, like Fire Dragons, Wraithguard, and supporting War Walkers and Fliers, and I think Banshees have a new lease on life.