Since Tarantino loves using Samuel L Jackson, why not imply that Windu turns into Vader?
That way you could still have the suit and the Vader hints if you’d like.
I don’t know how that’d be executed, but it sounds lovely!
I also feel like the Vader footage, if it’s present and not implied to be Anakin, would be confusing.
Characterize Windu as a Jedi who believes in the light side because he thinks that it’s naturally stronger. When he confronts Palpatine and sees his companions cut down effortlessly, he gains more respect for the power of the Dark Side. Anakin senses that Windu wants to assassinate the Chancellor and take his place, so he goes to stop him. Anakin here could be painted as a stalwart defender of Republic ideals. Anakin arrives just as Windu raises his saber to strike down the Chancellor, but then disfiguring lightning rains down on him. Palpatine screams about unlimited power. Scene end.
There are plenty of directions to go after this. One way to do it would be to show the attack on the Jedi temple without showing Anakin’s involvement. Show only one shot of a mysterious hooded figure, then the activation of a purple lightsaber.
Obi-wan discovers the massacre at the temple as Palpatine consolodates power. He has sent Vader to Mustafar to wipe out the Separatists, and we see brief flashes of a figure with a purple lightsaber striking down these enemies. Obi-wan fears that Anakin has done these things, and goes to Mustafar. It is here we see Anakin on that planet.
When Padme confronts Anakin, he explains that it was Vader that murdered so many, and he arrived too late to stop it. He further explains that he hoped to destroy Vader here, but he had already gone when he arrived. After he destroys Vader, he plans to capture Palpatine and put him on trial, and in so doing learn from him how to save Padme. Obi-wan then appears, and Padme collapses, perhaps in part due to the volcanic air along with the shock of Obi-wan’s anger. Anakin wants to bring her to the ship, but Obi-wan seems convinced that Anakin is a threat. He wonders just what happened in Palpatine’s chambers and how Anakin escaped the Chancellor unscathed. He believes that Anakin is really this Vader, and they fight.
In their final shouting match, Anakin laments how all the Jedi could turn against him and the ideals of the Republic, whereas Obi-wan blames Anakin for not helping Windu end the Sith.
Implying that Anakin is dead on the volcanic slope, and that Windu was disfigured from Palpatine’s attack like he himself was, would be all the justification needed for a mysterious masked figure to appear in Episode 4. Unfortunately, there’s still the problem of Obi-wan’s line ‘A young Jedi named Darth Vader, who was a pupil of mine before he turned to evil’. Though if this is only a fanedit of ROTS, you might be able to get away with the idea that Obi-wan taught Windu.