'Who means something and where are we at this point in the timeline with Vader?'
www.slashfilm.com
<snip>As we previously reported, Darth Maul was originally going to appear in "Obi-Wan Kenobi" as one of the main antagonists. However, Lucasfilm President Kathleen Kennedy intervened, had the scripts retooled, and it was Dave Filoni, the man behind "The Clone Wars" and "Star Wars Rebels," who helped guide them to Vader. Let us not forget that Filoni is the one who reunited Obi-Wan and Maul in "Rebels," providing a definitive conclusion to that journey. Bringing Maul into the fold would have, in some ways, contradicted what already exists. Vader, on the other hand, might be able to add something to the lore.
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I don't disagree that a storyline about Vader in the years between RotS and ANH would be good. I think they covered the Maul/Kenobi arc pretty well in Clone Wars.