It’s an interesting idea, but one that comes with a host of problems to solve.

1: How would Obi-wan be made to acknowledge that Grevious is his prior student? I suppose some introduction could be made during the crawl since that is also the case in the theatrical version, but it would need to be buttressed by dialogue from Obi-wan confirming their history. This would have to happen early in ROTS, since Obi-wan could only have taken another apprentice during the Clone Wars after Anakin had become a Jedi Knight.

2: If Grievous is already so far gone as to have a metal cranium, it would be pretty clear that this is not the character in the OT when we see the back of Vader’s head in ESB. There’s also the problem that Vader in the OT and Grievous in the PT have entirely different ways of moving, with Grievous being much more alien and animalisitic in his movements.

3: Obi-wan in ANH says that Vader betrayed and murdered Luke’s father. However, there is no one-on-one fight between Grievous and Anakin, and Grievous dies in act 2 long before we see the last of Anakin. If the film is cut to imply that Grievous survives, this would also necessitate Anakin surviving to be killed later by Grievous, but this introduces many more problems as well as leaving unclear how Grievous would ‘betray’ Anakin if he was already a darksider when they met (this is a problem with my latest Prequel concept as well, but it could easily be fixed by having Obi-wan’s apprentice meet and befriend Anakin earlier in their adventures).

These are the problems that I can think of off the top of my head. The frustrating thing is that Grievous has many aspects that would make him a good fit for this role, like being a cyborg with a raspy voice and being trained to kill Jedi, but the leap between that and Obi-wan’s fallen apprentice and onetime friend of Anakin Skywalker is just too great in my opinion. I’d very much like to be proven wrong, however, so if anyone wants to try and make this work all the power to you!