I have a sneaking suspicion that Tyrion will get a crossbow bolt to the chest this season, and that'll push Jamie to finally murder Cersei. Although, who knows. The show didn't actually use the "valonqar" bit from Cersei's visit with Maggie the Frog, so they may just ditch that altogether. All Maggie told her was that she wouldn't marry the prince, she'd marry the king, that the king would have 20 children, that she'd have 3, that gold would be their crowns, and gold their shrouds. And that was it. Then she just laughed at Cersei, and the flashback ended.
As far as the ending in the books and the ending in the shows goes, here's my bet.
1. The broad strokes are the same. Jon and Dany come together. R+L=J. The Night's King is defeated, the Lannisters are overthrown.
2. Any of the major characters who die in the show will die in the books. So, Jon, Dany, Cersei, Jamie, Tyrion, Bran, Sansa, and Arya are on that list. The rest are fungible (e.g., Brienne, Tormund, Jorah, The Hound, etc.).
3. The show has, as far back as Season 2, already changed the fate of various minor characters, such as people on Arya's list, Brienne's journey around the Crab bay or whatever it was, the death of Barristan Selmy (who's still alive in the books), etc., etc. Jorah is currently functioning as some kind of hybrid between Barristan Selmy and Jon Connington, and there's no "fAegon" in the show. I suspect that Martin's story will be much more circuitous, will have character behavior be much more "earned" and feel less abrupt in comparison to the show, and that likewise any ending he writes will probably feel more grounded and less like the accusations of "fan service" that the show has had leveled at it.
The "fan service" criticism, I think, is due more to the fact that the show is now very rushed and isn't taking the time to develop characters sufficiently to justify their behavior. They just...do stuff because it's the stuff that needs to be done, and although it satisfies the fans, narratively it isn't as well-supported as it should be. For example, Jon and Dany fall in love. Really? Based on what? Based on how much time together? Jon and Dany fall in love because they're "supposed" to fall in love, but the show hasn't demonstrated why their attraction and love is happening at all, other than the fact that (1) they're both very attractive people, and (2) they're...uh....supposed to end up together? So it feels like fan service. By contrast, if the show took the time to have their relationship together grow in a more realistic, believable fashion, the "fan service" complaint would be less of an issue.
Now, I'm sure some people would still complain it's fan service, merely because it provides a satisfactory and happy ending, but I think that's because people assume that to be edgy and surprising, you have to simply do the opposite of traditional heroic story structure, and, well, that's just dumb. It falls into what I consider the "Whedon trap."
When Joss Whedon started his career, his shows were revolutionary and groundbreaking. Main characters died! All the time!! But that was only shocking and surprising because we were squarely in an era where main characters almost always had "hero shields" especially in genre shows. So, killing main characters was surprising! It was daring! It was shocking! And then he kept doing it. Over, and over, and over, and over. Not only that, but he regularly would bring characters together romantically, and then break them apart, usually through some kind of trauma. Again and again.
The ultimate impact of this is that "shocking" deaths are no longer shocking. They instead become expected, and people start doing "death pools" or "character death bingo" and the like. We're seeing that now with Game of Thrones -- people are betting on which characters are going to die, and are often assuming the worst for many of the main characters. Moreover, they assume that the story must end in a sad manner or at least a bittersweet manner, simply because an actual happy ending will be such a departure from what's come before. And they criticize the show for hewing too closely to traditional fantasy epic structure, where the heroes win and save the day.
But honestly, I think that this has been Martin's plan all along. He's still doing fantasy, but he's trying to do it in a more realistic fashion. But the thing is, "realistic" doesn't mean "everything ends on a down note and the heroes lose." It just means that if the heroes win, it has to come at a cost, that it has to be earned, and that it has to make sense in context and not just happen because that's what happens in heroic epic fantasy.