Cersei has to have something up her sleeve. If this season rids us of her and Ramsey, who is going to be the new bad guy? Someone has to step up, and I don't see Ramsey making it out this year.
Also, I am super annoyed with Tyrion this season. He masterfully ran King's Landing, but he is making horrible choices with Mereen. I can't imagine Dany is going to be happy with what he's done. He effectively went against her wishes, brought in an unauthorized religion, and made a horrible assumption about the masters that ended up with him in a virtually indefensible position. He just comes across as clueless. It could be part of his arc... he gets Westoros, but not Essos, maybe? I dunno, but I am not enjoying it.
On the first point I'm thinking Euron will be our next big villain. I was reading a write up about how there's been three big villains representing different aspects of war or what have you. First was Tywin who was a very political evil, than Ramsey who was a very human evil and next will be Euron representing a supernatural evil.
On the second point I don't think he fucked up too badly. They didn't have the resources to mount attacks on the other two cities AND hold Mereen so they would have just dwindled what little they have until they were exhausted and finally defeated. Ask Sparta about that when they conquered Greece. After they were overthrown they were basically just a giant joke. This invasion was inevitable but he managed to buy Mereen some time to get its shit together and get the people united. That would help a great deal in a siege type position. However with Dany back and the Ironborn on the way I don't think its going to be a very long lived siege.
Little worried about Bastard Bowl. I think Jon will look crushed before the Knights of the Vale swoop in and save the day. Ramsay is being stupid meeting them in the field like Roose said which will give a huuuuge advantage to the armored calvary sweeping in and laying waste. Littlefinger will definitely use the situation to his advantage with his army stationed there and will basically force Sansa to be his puppet. Her machinations from that point forward are the wildcard.