Before I start explaining this, let me highlight one important fact that I think everyone has missed (unless I'm crazy): Savitar's real name is Dallas. We learned that in Episode 9 when we saw the future in which Savitar kills Iris. Pay very close attention at 0:10 in the video below. He says, "Please, Dallas, don't do this." Barry said Savitar's real name in the mid-season finale and no one noticed.
It has been interesting that there hasn't been much speculation about Savitar's real identity the way there was speculation for Prometheus, Zoom and a few other Big Bads. The writers of The Flash have taken things slow with that and we likely won't get his real identity until sometime after Episode 18.
Why Savitar (Dallas) Is Accelerated Man
It may sound crazy, but I'm beginning to suspect that Accelerated Man will actually end up becoming Savitar in the future. I have long suspected that Savitar might be from Earth-19 and even recently wrote that down. Considering the fact that Accelerated Man is the primary speedster from Earth-19, he would be a great candidate to be Savitar. Remember, Jay Garrick told Barry that Savitar would conquer Earths similar to Zoom.
The theory that Savitar is from Earth-19 has recently gained more evidence with the release of the synopsis of Episode 18, "Abra Kadabra" (Barry still won't know Savitar's real identity by then, which is crazy). The synopsis describes Abra Kadabra as a villain from Earth-19 who knows Savitar's identity. Abra Kadabra is from the 64th century, so if Savitar was from Earth-19, it would make sense for him to know Savitar's identity.
Listen closely to the video below showing Savitar's discussion with Team Flash. When asked when they first met, Savitar says, "Not long now." This suggests Barry hasn't met Savitar's present self yet and will meet him soon. Coincidentally, Barry also hasn't met Accelerated Man and will likely meet him soon.
Notice also how he hates on Earth-19 Wells. There's something personal about that. Almost as if the "fake" Wells is from his Earth.
He also says that Barry "was there" when he became who he is. It's possible that Accelerated Man will eventually undergo a radical transformation at some point in the future.
The purple lightning is another reason why he is likely the man who will become Savitar. We have never seen that lightening before and it was clearly intentional.
In the comics, Accelerated Man's identity is unknown. From my understanding, he rarely appeared in the comics, which could mean that the writers will mess around with him a little. His real name could definitely be "Dallas."
Savitar Comes From The Future-2056
Savitar looks pretty teched up, so he probably comes from decades into the future.
In that video most people assumed (as it was implied) that 2056 Barry Allen was referring to Flashpoint. But why would he be talking about Flashpoint 40 years into the future? If the Legends visited him there, the changes from Flashpoint would already be in effect.
Instead, I think it is likely that Barry Allen from 2056 traveled back in time and changed things. He probably time-traveled to stop Savitar and trap him in the Speed Force. Savitar has said that Barry's future self trapped him in the Speed Force. After traveling through time and changing things, 2056 Barry felt obligated to inform the Legends.
It also seems possible that Flashpoint didn't actually cause the changes to the timeline that we have seen in Season 3. Logically, those changes shouldn't exist because Barry returned everything back to normal. What could have caused those changes is 2056 Barry trapping Savitar inside the Speed Force.
Inside the Speed Force, Savitar likely found a weak spot in Barry's past: Flashpoint. Seeing the transition between Flashpoint and the timeline Barry came from, Savitar realized that he could blindside Barry by manipulating events and creating the circumstances needed for his return.
Trapped inside the Speed Force for an unimaginable amount of time, Savitar was likely able to see into the future — a future that he meticulously crafted, and one that relied upon Barry seeing the future in order to affect his actions.
Why Does He Kill Iris?
Savitar has repeatedly said that he is "the future Flash." In order to be the Flash of the future, he needs a Big Bad. He also wants everyone to see Barry Allen the way that he does. In order for that to happen, he has to make Barry evil. Killing Iris could create the circumstances for Barry to become evil.
This is why he will not kill Barry yet. He wants to be the Flash again, like he once was on Earth-19. After Barry took that away from him (in his mind, Barry took that away from him. In reality, he likely took that away from himself), Savitar wants to take it back. Savitar is willing to do anything to get that, even if it means prolonging the life of his mortal enemy.
Should You Believe Me?
No, definitely not. I am just writing theories like this for fun in order to try to explain everything going on. Remember, and I probably shouldn't remind you, I did incorrectly suspect that Prometheus was Konstantin Kovar (but I will definitely argue that Kovar would've been a much better choice). It's very likely that I am wrong again, as these things are hard to predict, so take this lightly.
He could also realistically be Julian, but it's hard to imagine fans being OK with them pulling another Judas with the Big Bad. The Accelerated Man storyline above is much more interesting. Hopefully this is at least a fun theory.