It's a wise choice doing it every Saturday, and I agree with others that I suspect something new must be in the pipeline. I'd say Scary Tales and Zombies will return, but both mazes (Terror and Sanctuary) will be changed or have their stories changed.
I'd say the Scarefest Saturday thing has probably been chosen because of the Big Six not continuing, and so marketing are probably looking at the events (Scarefest & Fireworks) to, as already stated, boost guest numbers and to round off the year with a positive, so people return the following season. It seems they are really pushing out on Scarefest this year, and I hope they make a mini-site again with the adverts for the mazes, the games and the countdown calender, as those were extremely effective in building up to the event in my opinion, all of which seemed to of not returned last year or the previous year, where both events seemed half-arsed with limited enthusiasm from marketing or entertainments, more so last year than any year.
But with Scarefest Saturdays, it seems that they really want to push the event out to make people go back again. The fact on its own things are already planned and ready to be revealed suggests they are more prepared and things have been approved (such as their main plans, for example a new maze or such), probably because Merlin & higher-end managers are desperate for the guests to return so will approve anything for the events or give a bigger budget, which is good for us.