I understand that the poor character has been dragged through the mud so many times that people would barely recognise him as he was in the original novella, but this seems to be a new level of alteration entirely.
What could Henry Jekyll possibly have to do with the world of resurrected ancient Egyptians?
He's a member of an organization called Prodigium, an organization that's been around for some time studying monsters, and seemingly also hunting and capturing monsters as well. He's the character who introduces Nick Morton (Tom Cruise's character), as well as the audience, to the fact that monsters have been roaming the Earth for thousands of years.
It seems that a major theme of the film (and maybe even the whole franchise down the road) is the balance between one's inner monster and their humanity, with some characters leaning one way or the other. For instance the Frankenstein Monster has more humanity than, say, the Invisible Man, but ironically the former is more blatantly a supernatural creature than the latter is.
This duality is usually metaphorical but a character like Dr. Jekyll, and his alter ego Mr. Hyde, literalizes this theme and provides a point of contrast with both the Mummy herself and the now cursed Nick Morton.
No problem. Other things worth noting are that Jekyll is older than he appears, so I suspect his origin was more or less as we'd understand it, taking place in the 19th century just as we always thought it would.
As I mentioned before Nick Morton is cursed, but I suspect that his curse will lead to him becoming a mummy himself. This places Nick Morton in the role of a pseudo-Larry Talbot, meaning he's a doomed protagonist fighting a curse that is turning him into a monster. In the event that my speculation is correct and Morton is becoming a mummy, this makes the duality theme resonate that much more.
So in a sense we have three examples of monsters with different forms of duality. There's Ahmanet, whose duality of human and monster is seemingly more a matter of personality, putting it more in the realm of metaphor. There's Jekyll and Hyde, a singular figure who moves back and forth between two different personas. And there's Nick Morton, who (if I am right) is transitioning from one form into another.
So in addition to cluing us in that we're in "a new world of gods and monsters" Dr. Jekyll provides a sort of foil to Nick Morton, and possibly to Ahmanet as well. Ahmanet is captured by Prodigium at some point, while Dr. Jekyll works as part of them. Why one monster works for them and another is captured by them is something I would expect to be explored at some point, either in this film or down the road in other films.
Since you clearly are knowledgeable about this, how do you predict the franchise will keep Tom Cruise active in it (I can't see him throwing on the horror make up for multiple films myself)? Or do you think this will be a one-off thing?
I wonder if Hyde is a result of Jekyll's attempts at creating a monster they can control or if he was already a monster when he was recruited. Maybe they have a way of keeping Hyde in check.
Also on a personal note I hope Hyde is closer to his original version. I love what they did with him in League of Extraordinary Gentleman (comics) but he doesn't have to be this universe's Hulk.
I'd advise you to create a mental wall between the novella and this adaptation. I doubt it's going to be faithful, but it's always possible to accept it as its own adaptation.
With that being said, the other posted summed it all nicely. This is a new cinematic universe, that's setting its own continuity that's inspired by all of these old stories.