Please remember that these are rumors, and some salt is required.
From the Beasts of War, we have it that the random game length is a thing of the past. At least for the most part, meaning that in most missions there will be a set number of rounds. There may still be a mission or two still sticking around that still incorporates a random game length, but for the most part they are a thing of the past.
Another thing discussed is a mission generator. Now Beasts of War thinks that old leaked 6th edition rule set was indeed an early playtest from GW. In the back of it was a large mission generator, which was a very cool way of generating missions, and there is a lot of hope that we do indeed end up with something like this.
Do you like random game lengths? and what would you like to see in a 6th edition mission generator?