Most extreme example: What would someone who watched Avengers Endgame have gotten out of it if they hadn't at least watched Infinity War? Likewise Wanda's Face-Heel turn would be totally out of the blue for someone who hasn't seen WandaVision.
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I think Avengers Endgame works stand-alone, actually, pretty well.
A part of that is due to the lengthy build-up, where the effects of the snap are both described and shown quite clearly.
I think this works better for AE than it does for MoM because the primary effect is external, and relatable. You could substitute any world-changing effect, such as a limited nuclear war or bio-weapon release and have a similar outcome. One doesn't have to reach very far to understand what happened and to see the consequences.
In MoM, on the other hand, the effects are internal: The conversion of Wanda to her unbalanced, misdirected, self. While I would argue the effect is simply not shown well enough, there is an extra difficulty in that the effect is internal, which makes it harder to understand. We need a lot more character development for Wanda.
(I personally think that the story-telling in MoM being not as good as in AE makes the story work less well, stand-alone, but that is a different matter.)
I have a question: Why would Wanda want to raise the children from a different Branch of the Multiverse? While the characters are similar, they are definitely not the same. Why would she want to displace an alternate version of herself to raise children who would not really be her own? If she was afraid for their safety, couldn't she act as a Multiversal guardian?
Or, wouldn't there be some Branches where Wanda was killed, and the children left as orphans?
Wanda: "Hey, America, my alternate died in Branch XYZ, could you send me over there so that I could look after the children she left behind?"
America: Sure thing!
TomB