I've seen many post what they think will happen and express what they want to happen in this episode, the last of the entire show, so here's a forum specifically for that.
There's quite a bit that needs addressing and/or wrapping up before the show comes to an end:
Mary desires to "build bridges" with Edith - certainly after the last episode at long last she finally seeing and treating her in a better light, and it seems will continue to seek further reconiciliation and friendship. I know myself and many have expressed the opinion that it should be Mary who brings Edith and Bertie back together to fix what she did (which leads to the next point).
Edith apparently has "nothing left to lose" regarding Marigold's true identity, but it still poses a threat - well we know for sure now that Bertie is returning and there will be scenes at Brancaster. So I would say there is hope he and she will get back together, especially if that report of a prop left behind is to be believed. As for Marigold's identity being a threat, my guess is that will come with Bertie's ambitious and stern mother. Bertie it seemed had no problem with Marigold being Edith's daughter other than he was hurt she didn't tell him, and Edith felt he would have come round about that. Bertie also mentioned his mom would be "thrilled' at him marrying an Earl's daughter. But if she somehow finds out about Marigold, she might try to sabotage any attemps to bring Bertie and Edith back together, sort of like how Susan tried to sabotage Rose's engagement to Atticus last season.
Henry's going to have trouble finding his place at Downton, as Mary's husband and George's stepfather. I wonder if this will have to do with his love of racing. Could this be one character who doesn't get a happy ending? I wonder, since the wedding felt rushed.
Carson is going to have some issues which prove even he is not "impervious to change". These too I can only imagine, but they might involve his wife or perhaps the way Downton is run. Certainly he was not happy when Robert considered downsizing the staff.
Rose, Atticus, and Shrimpie are returning, and Anna for sure is pregnant. So we're all hoping for a Baby Bates, and some are speculating perhaps Rose is prengant too. Could we see more than one birth? Well the episode is longer than any other at two hours, so perhaps they can squeeze it in.
The length might also be enough to squeeze two weddings in: Bertie and Edith, and Dickie and Isobel. I imagine the former would be the big public one, while the latter might be quieter and more private. Isobel said she would only get back together with Dickie if she heard from Larry himself that he would want it. Now, I don't know if this is confirmed yet but something on Tumblr suggested Merton is going to have a clash with his son. If so, and with Larry, in my opinion it's about time. From the beginning I've wanted him to disinherit Larry; I'd rather see Tim Grey inherit since he's the lesser of two evils.
Barrow after his attempted suicide needs something to cheer him up, perhaps a desk job at the hospital. I wonder what Daisy will do now that she's done well with her exams. Mr Mason wanted her to move in with him; he might still want her to, so she can inherit his current tenancy.
Some want to see Laura Edmunds hitch up with Tom. While he might or might not, they could be a good pair - that might be something that's left for us to wonder, something that has the potential to be. Tom did say he wanted something to do that didn't have to do with the estate. If not cars, he ought to rekindle his interest in politics.
Anyway, those are my thoughts. I might have more in the future and leave them in the comments. Express your own too!