My basic theory is this, all the important phases of Anakin's fall are in ROTS. Lucas said the PT was the story of the rise and fall of Anakin. But we don't really see his rise anyway. He never becomes what he could have or should have been as a Jedi. Lucas should have started the story much later in Anakin's life and focus on his fall and just ignore his "rise". Start it with Anakin as a 25 year old Jedi, the best Jedi in the Galaxy.
This is kinda where ROTS is. Think about it. The first part of ROTS is Anakin and Obi Wan on a mission, the mission is to save the Chancellor. The mission is successful. The 2 of them are out there having fun, joking with each other, R2 is destroying battle droids. It's just a fun adventure. Kinda like ANH and TFA. Obviously you'd have to flesh is out more but in reality you could make a whole movie about the 2 of them having to rescue Papatine, they made a whole movie about Luke having to rescue Leia afterall.
The 2nd part of ROTS then is the main characters separated. With one going on more of an action type story and one staying back and learning. Basically like ESB and TLJ. The movie could easily culmin ate with some form of the Palpatine, Anakin, Mace scene as the climax or maybe even Anakin's attack on the Temple.
Then the last part of ROTS is a last ditch effort to save what's left of the Republic and maybe bring Anakin back.
Basically everything fans wanted or expected to see in the PT is in ROTS. It just mostly isn't given enough time to be done right because its all compressed into one movie.