I still think Steve has very high chance of being in.
It's still in the top 5 eshop sales of last month, he have a game coming in May, and honestly Minecraft is just too big to be ignored. Sakurai said that Smash ultimate is basically the video game crossover now, and you can't pretend to be the video game crossover and ignoring Minecraft.
As for Chief, honestly I think his chance are pretty low because Halo was never that big in Japan and he have no presence on Switch. People start supporting it a lot because of that rumor about Master chief collection on Switch, but we haven't heard anything new about it so it might be fake.