I want to start a discussion about Savitar and who he might be.

Personally I think he's not from this earth or any earth at all. He's after Barry, as all villains do, but after the events of Invasion! I think he might be a Dominator. Look at his hight, his head and his suit. All of that we have seen in the last episodes of Invasion!. The Dominators tech, intelligence and appearance. It might be that one of them wanted to escape and got stuck in the Speed Force for a while (maybe for 65 years?), made himself a hightech suit drained with Speed Force. That might explain why he's so fast that he appears to be invisible for the human eye or even Meta-eyes and only the ones who understand the Speed Force can see him. The Dominators were after Barry because he screwed up Flashpoint, so my guess is that they can notice the differences between timelines and that's why Savitar is using Alchemy to bring back Meta's from the different timelines and earths.