Regarding the death of Gluttony:
In order to figure it out, you'll have to have read Chapter 52. Explanation inside the spoiler brackets:
If you've read chapter 52, then you know Envy is sort of like a snakelike/reptiliian-style monster in his/her real form. Gluttony has swallowed Envy, Ed, and Ling. When Ed is swallowed by Envy in chapter 52, he sees a Philosopher's Stone and a Gate of Knowledge inside Envy.
My speculation is that Ed is going to use the Gate of Knowledge inside
Envy to rip straight out of Gluttony's "stomach", i.e. the False Gate zone. In the process of exiting through this opening of the Gate of Knowledge, all of the False Gate zone will be sucked through; literally tearing apart Gluttony from the inside.
Since the Divine Comedy lists Gluttony's death as related to eating snakes and rodents and filth; this would be appropo - Envy is the snake that he swallowed, and it would ultimately be the Gate inside Envy that will be his undoing.
If you think about it - it makes a lot of sense. Ed, Ling, and Envy will end up imploding Gluttony right in front of "Father", presumably reuniting them with Al, Scar, and Mei-Chan - which would set the perfect stage for the next step of the story.
[/spoiler]Regarding the death of Envy:
On the Chapter 52 thread, Tyoria suggested that Envy's undoing will be her eyes, as depicted by Dante's version of the punishments. She backs up this theory with an overwhelming amount of evidence to lend credence to this idea as a serious explanation. See her(?) line of reasoning here (beware of spoilers): Tyoria's "Eyes" Theory
. That offers some compelling evidence that the "freezing water bit" may not happen and may very well be more of something along the lines of this instead (spoiler enclosed for those that do not wish to speculate):
[spoiler]Envy will have her eyes "sewn" shut (or burned or sealed or something of that nature),, and then she won't be able to morph her form anymore - the literary equivalent of "being forced to wear clothes that look like the ground."' Stuck in her "fragile" androgynous homunculus form, she wil be easy to defeat in physical battle and will basically be killed.[/spoiler]
Again, this is all speculation on my part (with credit to Tyoria for her lead on Envy), but I think they are the most likely theories for the fates of Gluttony and Envy. As for Wrath
(the only other homunculous we know much about), I have little speculation at this point, other than to believe that being "dismembered alive" may be the equivalent of Scar alchemically deconstructing his entire body (thus separating the physical matter from the Philosopher's Stone, preventing it from being affected by its regenerative powers and thus preventing Wrath from recovering), or
Zolph Kimbley doing something similar by obliterating him as a "man-fueled bomb." Those are long shots, but that's the best I can think of.
Speculation on Sloth or Pride? I've got nothin'. Comments are welcome.