Brand New Day no longer needs to address Doomsday's event, and can happen in parallel to Thunderbolts*

Maybe even taking place in the gap months before the post credit scene

I could be mistaken but Pearson didn't say that about Zemo. He said it was only an idea that never went to light because it would be too obvious and not a different twist that surprise the audience.

I suspected Wanda would be #1.

When I finished watching Thunderbolts* the first line my friend said to his brother was, "I think Wanda would adopt Bob as her son or new brother right away if they meet."

It sounds silly, but I think makes sense. Wanda could sense what struggle Bob had gone through, and would instantly relate to.him. And Void's power is meaningless now that she has become Scarlet Witch. The 838 Wanda wasn't even her full power but could wipe out all but one Illuminati (Mordo). Full power Wanda would have no issues inside the Void's mental labyrinth.

I think Eric Pearson said Bill Foster was initially gonna be in the movie. But Ava's planned bonding moment with Antonia was changed. Maybe that decision rippled into eliminating Ava's backstory altogether.

The Russos directed the post credit scene. So, it's likely a longer scene in Doomsday, much like Civil War's teaser. That's where the answer will be found.

As for why Bucky didn't call Sam?

1. Has Sam's wing been fixed at the time? It was ripped in Captain America Brave New World, right?

2. Maybe Bucky knew Sam wasn't nearby, recruiting team . Most likely in San Francisco for Scott Lang and Shang Chi.

3. Sam clearly rejected an Avengers under government supervision. And Bucky was out gathering evidence (Yelena, etc) for a government matter. Sam helping him would risk being called interferring with government work or even considered federal crime. This may also explain why Sam never New Avengers, which is run by government.