brimforge said:

For the "big secret" - one could leave the "Vader transformation" offscreen - that isn't so bad ... on the other hand, we could lay some red herings, like turning Darth Maul into General Grievous (I try to make some mockups how "Darth Grievous" could look onscreen)

An offscreen "transformation" would be the best way to handle it and preserve secret I agree.... you could keep things relatively simple and not have too many decoys thrown into the situation.

I could see a scenario where Anakin and a group of other Jedi are sent off on a mission together. Later, Obi-wan is informed that "one of" his apprentices has turned to the dark side... Obi-wan responds to this in to something along the lines of "Anakin, is he ok?" only for the informer to simply say "he has fallen" and he hands Anakin's lightsaber over to Obi-Wan, making the audience think he is dead.

We could then have a faceoff with a masked, red sabered Vader (not yet mechanical).... he would be defeated in a similar fashion and reconstructed in a similar fashion that we see in the end of the RotS... no names of Anakin being used, but some emotional lines could still be spouted between the two, but the audience views this battle as Obi-wan going after the fallen jedi who killed his best friend and "brother"