Eh, regarding the scene in R1, Vader clearly brings troops with him. Its evidenced by both his line about preparing a boarding party and the fact that Stormtroopers show up next to him as they watch the Tantive IV fly away. As to why he takes on the rebels single handedly? Perhaps he had his team split up to search the ship faster and he just happened to be the one who found them first. He didn’t really have time to wait for backup. It’s an entirely different scenario than the opening of Star Wars where he’s cornered and captured the ship and can take his time letting the troops storm it.
Now, as for my film idea, I do think the fear of Vader not being able to carry a film is a valid concern that must be dealt with. But I don’t think it’s an insurmountable problem. We already know all we need to about him; he needs no character development. However he can still be the protagonist in that he is the catalyst and the driving force that propels the film. But even so, the focus of the run time would actually be on his victims. Getting to know who they are, how their covers were blown and the Empire finds them, how they deal with the fear of knowing Vader is coming for them. Again, this is roughly similar to how things are done in The Sword of Doom.
Also, I had an idea for a scene (perhaps the opening of the movie?) where Vader commands the team that ransacks the Kyber temple with the monks who would later seek him out watching in horror/terror. Perhaps they vow to avenge the desecration and go hunting for Vader, tracking him each time he hunts down another Jedi until they are able to confront him. Maybe this would be the big finale. Vader against a team of force sensitive (but NOT Jedi!) Guardians of the Whills. Or maybe it’s just the opening scene and the monks attack on site and overwhelm the Stormtroopers until Vader steps in.
At any rate, while it would be a “Darth Vader movie”, I actually envision it having Vader onscreen for not much more than he’s featured in ESB. I think it could really work. Man, the more I think about this, the more I REALLY want to see it made! Anybody happen to have an in with John Knoll or Kathleen Kennedy…?