MCU Theory: How The Original Black Captain America Changes Everything
1. Zemo, The Government & The Super Soldiers
The reason for Zemo's reappearance makes a lot of sense in this context: he discovered the secret of The Winter Soldier programme and its undiscovered super soldiers (who he killed) and his disdain for Captain America and the idea of government sponsored superheroes with no accountability could make the value of a secret like Project Rebirth even higher. In his hands, it would be a weapon to bring down the entire government.
If you think about it, Zemo's hatred of Captain America as a symbol sets him up perfectly as the right person to destroy his legacy. And that is something we've already seen in the MCU in Spider-Man: Far From Home, in which Mysterio's real plan was to discredit Tony Stark and destroy HIS legacy. For them to revive that idea would fit their usual approach to using similar themes across multiple stories.
And making Captain America a product of an evil scheme with secret victims would absolutely fit.
With the story focused on the secret of the first Captain America and his fellow test subjects, we could see Sam and Bucky tracking Zemo after his escape, discovering the secrets of Project Rebirth and realising that the US government are the real villains.
That would set them on a path of clashing with John Walker's new Captain America and his government sponsors as they attempt to reclaim what that mantle means and change the perception of Steve Rogers' legacy. That would also make Walker the villain of the piece, which we all suspected when it was announced that US Agent was going to be involved at all.
Having the rebranded US Agent take the mantle of the new Cap is important here too, because it will fit with Sam's personal crisis over the meaning of being Captain America. Walker is no more than a jingoistic symbol doing the government's bidding - he's The Winter Soldier with an acceptable face - and his militaristic attitude and inevitable over-enthusiasm will make him the opposite of what Steve Rogers stood for.
Sam and Bucky fighting him to reclaim the Captain America name is a hugely significant symbol and calling back to the man (or men) who almost lost everything so that Captain America could even exist is a big deal too. Giving him his name as Sam seeks not only to clear Steve's but also find his own is perfect. Having him reclaiming that history and putting respect on Bradley's name would be perfect too.
In short, Isaiah Bradley is going to be the key figure in convincing Sam Wilson to become Captain America and by the time the show finishes, we will legitimately have a new Captain America.