My overall theory is that Savitar wants to replace Barry because as he said in the Christmas episode, “everything you took, I want it back.” So with that in mind I went back and re-watched a few of the past episodes I picked up on a few things that may have come back into play in order to save Iris’ life.
1. There is the face changing technology that suddenly became the whole body changing technology. And for an added bonus we got to see that a man could totally become a woman and be 100% believable.
2. The metal spear/rod thing from Savitar’s suit that they yanked out of Barry’s shoulder. The same metal spear/rod thing that Jesse Quick used to locate Savitar. The same spear/claw thing that HR was checking out the last time we saw him.
3. And then there’s Savitar’s suit that is so much cooler than Barry’s. The same suit that can walk around and fight all by itself as if has has someone actually in it…
So my Iris survival theory is this: I think Savitar has been planting the seed since his very first interaction with HR that he’s weak, a coward, useless, etc. because he knew that eventually HR would do too much in order to prove him wrong. Which is why when Savitar had his fight with Barry in Wrath of Savitar he left a souvenir in Barry’s shoulder. A souvenir that is more or less a GPS system that leads someone right to Savitar.
In HR’s last scene it was still about 20 minutes before Midnight so he had plenty of time to take the the spear/rod thing and track down Savitar. I think first he’ll attempt to save Iris (who will probably be somewhere in Savitar’s lair) but he’ll get caught basically because Savitar was expecting him to come. He will be given a choice to save Iris or go free or something like that and he’ll choose to save her. He will use his face/body technology to become Iris AND Savitar will use it to become HR because he remembers HR being there because of them seeing him in Cisco’s first vibe. Meanwhile Barry’s “super cool” works-on-it’s-own suit can take over the actual killing of ‘Iris’.
Since the only way we’ve seen the face-changing tech work is when the pen light (or whatever it is) is shined in a person’s face odds are if 'Iris’ is really HR she’s not going to morph back to him on her own.
And that’s where the engagement ring comes in.
Savitar has reiterated over and over and over again that Barry needs to see Iris die in order for him to live. Barry has to believe that Iris is dead because that’s what drives him to do whatever it takes to defeat Savitar–including creating time remnants. It’s been proven that Savitar is a master manipulator and he can make someone believe what he wants them to, which is why he’s viewed as a God when he’s nothing more than a man in a suit. I don’t think he wants to kill Iris because he is still Barry and does still love her no matter how twisted and evil he is. What he wants is revenge on Barry because he hates him. He wants him to hurt and be heartbroken and lose Iris. That, IMO, is his plan.
The thing about it is Savitar has probably considered every possibility and dotted every 'i’ and crossed every ’t’. What he couldn’t have predicted was that Iris would take off her engagement ring and give it to Joe. Her not wearing it when Wally was vibed to the future was a big deal at the time BUT once she and Barry engaged again it became a moot point–so of course she would be wearing it in the future.
So in the process of mourning Iris, Joe is going to see her wearing her engagement ring. No one else will realize the significance of that until he pulls it out of his pocket and he’s holding the same ring that 'Iris’ is wearing. That’s when they’ll realize that the dead Iris is really HR and Savitar has the real Iris captive and that’s when they’ll put a plan in motion to find them and save her.