Maybe, like in the comics, Wanda’s children are made from Mephisto (or something like that) and so he kills them?

Yes, Wanda got shards of Mephisto's soul to make them.

The wine bottle Wanda has in the trailer reads Maison du M[] (forgot what it really read), if that helps.

It reads "House of M" which is a Scarlet Witch centric event in the comics where she alters reality.

Yes, I watched a YouTube video which explained the Wine bottle. It’s a nice Easter Egg.

Oh hell yeah. I think Mephisto is a great villain and one who could continue beyond WandaVision with Ghost Rider and particularly with Doctor Strange and Doctor Doom. One of my all time favorite Marvel stories is Triumph and Torment, which features Mephisto. I think that would be a cool way for a future Doctor Strange movie to go.

Maybe he could be in Dr Strange 2?

Who or what is mephisto?

Sorry I am not very updated in comics

@IceMoh ... Thank you

Is he more powerful than Thanos?