regarding who will be up to speed by the end of next season:
-jamie: not much to do except take river run back since he already set Brienne out
-sansa (she's already there, right?): The last she does is come down from the Eyrie so they need to create something for her to do
-bran: he gets to the caves and starts looking through the trees, not much to do
-brienne: already set out and met Pod, just needs to meet up with the bloody mummers and Thoros's crew and Stoneheart
-arya: she has a pretty good story arc in Braavos but it can all be resolved
-sam: goes to the Citadel, not much else
-theon and the boltons: already took Moat Cailin, just the wedding and Winterfell
-jon snow: not a lot left either there, they changed up so much little things but again, not a lot to do except manage wildlings coming across the wall
-stannis: go to wall, leave wall, blizzard, yawn.
-dany (please please please skip the martell suitor): there's not much left for her to do but I think they will need to pace it for Tyrion's sake considering there is the big war in Meereen in WoW. will probably see less of her or just invent something as filler, I'm trying to think of a bigger budget scene though than Drogon in the fighting pit
-tyrion (mereen is a long way for him to go in one season, but if he doesn't get there next year, how the hell are they going to stretch the dany stuff out? playing the "who should i marry" game for 10+ episodes isn't going to cut it): agree, Pentos and the journey down the Ryhone should be enough, there's a lot to do with Griff and Aegon and flushing that out and it's going to be important. End his story with Jorah finding him at the brothel and then you have the trip to Mereen and slavery, et.
-cersei (though i think they'd do well to cram her whole arc into one season. otherwise, king's landing is going to be agonizing to watch).: They can cut a ton of filler. They can highlight her stupid decisions with one small council meeting, she plots to frame Margery quickly, don't need a lot of detail, and then her problems with the High Septon. You can probably devote 10-15 minutes an episode on average to the crap in Kings Landing and cover all of it adequately.
and regarding the cersei story...i suppose there's little doubt who robert strong is at this point.: that would make a decent ending to the season, the walk of shame and Robert Strong standing there
Overall though, everything can be sorted out except for Mereen only because Tyrion does so much without a lot of text because Martin doesn't add stupid, useless filler. and if you don't get him to Mereen you can't finish Dany's story in one season.
Not sure how much they will focus on Dorne, I think they are a red herring for now, unless there's something huge involving Myrcella they can skip all of that stuff.