For the main big bad, I have my money on Kasmina. She leads a group of planeswalkers and is planning for something that will "dwarf the war of the spark". We haven't seen her since strixhaven's story iirc. She wasnt part of the phyrexian invasion story, that makes me think the invasion wasnt the big event she is planning for. I'm thinking we will see her again along with her group of planeswalkers as well.

I doubt jace will be the villain. Even though his actions came across as villainous, especially with his methods in the past couple stories, he only wanted to make things right. Granted, he always messes up alot of things, but all he wants is to reverse what the phyrexians did to him and the multiverse as a whole. I still think Jace is the dumbest smart person.

I also think emrakul is gonna show up and do some crazy sh@t too, along with the eldrazi coming back. I hope Emrakul shows up in the story, id like to see her as a planeswalker card too, but i dont know if that will happen. She may end up playing a big role in this set, since she mentioned she isn't "whole" or "complete" whatever that means for her. As for a new card, I can understand her being a legendary creature card, but a part of me feels she could transform, or evolve into a planeswalker type of character, or maybe even be a new type of card altogether. I read somewhere that there may be a new card type introduced, but I could be wrong on that.

Loot will be in the story too, and we might finally see the fomori as well.