"Roy and Riza in the movie" . . . le sigh.
I'm sorry, I'm working on a fiction that pulls from the movie for its startpoint. I have had to do BACKFLIPS to explain what Roy was thinking, haring off to the North and whining for two years. So this is sort of a subject-of-momentary-angst for me.
As for the original subject of the post:
1) I don't want her to die. So I am going to assume she's going to, and refuse to get my hopes up.
2) From the author's perspective, this is the time when the stakes need to go up dramatically. Death is a good way of introducing tension. [spoiler]To put it in perspective: Hughes died in 15. Greed and Co. died in 31-ish. Ling <arguably> died in 54. Each death has ushered in major plot twists and significant diversions --and in fact, we're due for another. I'm voting for Scar.

[/spoiler]
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All that said --I don't think Riza's going to die. Yet, at least.
I agree with Summoner Colette that Riza's death would aggrandize her character out of proportion. She is a protagonist, not -the- protagonist. Martyring her now would be counterproductive.
Attendant on this: Riza is incredibly useful where she is. If the author has information she wants to sneak us about the Homunculi (and especially if she wants to work on our perceptions of Bradley) this is the natural avenue for that information.
Similarly, Riza is a better hostage than she is a corpse. I know this has been said, but I don't think it can be emphasized enough. As long as Bradley has Riza, he has Roy's measure.
Right now Roy's goal is beyond his reach, and so he's can still afford to be moved by emotion. When Bradley pulls the "Hawkeye" string, then, Roy responds. But if the Flame Alchemist gets within spitting distance of overthrowing the Homunculi, do you really think that he'll falter at a threat to Riza? (And do you really believe Riza would allow him to?)
If Bradley wants to know what Roy's up to, all he's got to do is exert a little pressure on Riza. Depending on how quickly Roy responds, Bradley will be able to gage his progress.
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Anyway: I think we're all in agreement that SOMETHING will happen to Riza. I'm more interested in WHAT.
There *are* fates worse than death. Being Bradley's assistant is not one of them. It is dangerous, but in a passive, uncomfortable way.
I can see her being maimed, I guess, but that would just seem to be a repeat of the Havoc incident.
A much more interesting tactic would be to turn Riza to the "dark side" --either by causing her to have doubts about Roy, or Homunculizing her. (As we saw with Bradley, nobody said the transformation was voluntary.) I dunno that I like the idea of her as Gluttony, but as Lust? You gotta admit, it'd be interesting. x.x
I have to admit that I
really do not understand where all this speculation about Riza being used to chivvy Roy into opening the Gate comes from.
My understanding is that the Homunculi want gate-openers for two things: Sacrifices, and the final person who will be directing/focusing Xerxes-esque Transmutation that will cast God down. (Or whatever it is, quite, that they're trying to do.)
I'm assuming that Father will be at the heart of that "Devour Amestris in a Night" vortex. So . . . they need another potential sacrifice, and Roy seems likely to survive the experience. I'm fine with the reasoning to there.
I don't understand how Riza has anything (really) to do with that. By now, Ed and Al know that a dead human
cannot be brought back. There's no indication in the Manga that a Philosopher's Stone makes it possible to revive the dead, either. I don't know if Ed and Al have shared their discovery with Roy, yet (they did have a prolonged car ride where they talked about alchemical things in 63). But I can't imagine that they
won't tell him. Maybe when they get back from the north.
In effect: I have no idea why Roy would try to Transmute Riza back to life (especially, but not only if, he knew that the effort would fail.) "He loves her" isn't a sufficient reason to me. Roy knows what she expects of him, what Hughes died to promote. If he died, or seriously injured himself in a failed transmutation, he would be wasting all of their efforts. Riza really would KILL him the next time she saw him.
Just my meanderings on the subject, anyway. ^^
-Arantzain