This is hard to predict, but here is my reasoning:
They are not going to have anyone cash in on Brock, and Brock could hold onto the title for another year if they want him to. Therefore, they are not going to have a RAW superstar win the Men's MitB match.
They are also not going to have both MitB winners be from the same brand. Therefore, a RAW female superstar will win MitB.
Ronda is most likely going to beat Nia for the belt. Since Natalya has been introduced in kayfabe as a close friend of Ronda and as a face on RAW, it would make sense to have her win the title, and cash in on Ronda straight away, turning heel. Therefore, I think Natalya is going to win the Women's MitB match.
Now the Men's winner is harder to pick, as all 4 Smackdown guys have a good shot. But I think they want to keep New Day in the tag title scene/sometimes compete in the US title scene, not the main event scene. So they are out. I think they think that Joe can be a contender at any point, so they won't pick him to win either. Rusev Day is popular, but is it main event worthy? I think management thinks "No", therefore Rusev won't win either. This leaves Miz, who is a perfect candidate to hold the briefcase. Imagine the arrogance. It is believable that he would need the briefcase to win a main event title off of the talent on Smackdown Live. And him winning will make him a 2-time MitB winner (putting him on the same level as CM Punk, and more importantly, CARMELLA :emoji_slight_smile: ). Therefore I see Miz winning the Men's MitB match.
They're my reasons.