I feel more than confident in saying no one cares about a single person from 838. Their Wanda has no personality, her kids have no personality and act younger than they are, their Christine has even less personality than the regular one and their superheroes are so uninteresting that the whole point is nobody in the audience cares that they all die.
Can you imagine a What If? set in that universe? Yipe.
Anyway, I think the "reincarnation" idea from the comics is still the likeliest. I thought they would avoid that hornet's nest, but then I thought they would avoid doing Avengers Disassembled. But nobody cares about 838 Billy and Tommy, they want the Billy & Tommy from WandaVision, with their personalities and powers and relationship to our Wanda. So they'll probably do the thing where there are these teenagers who mysteriously resemble Wanda's supposedly imaginary children.