With Pyra and Mithra now revealed, I am convinced that the last two charcters will be big hitters. Even as someone who doesn't know a ton about Xenoblade, I still am very happy for fans of Pyra and Mythra and appreciate how Sakurai went out of his way to add Xenoblade 2 and ARMS characters after not being able to include them in the base game despite his want to. However, at the same time I also recognize that this was a much safer, less wild pick then Steve or Sephiroth. Those characters were jaw droppers that many would have never, ever anticpaited would join the battle a few years ago.
They way things are lining up, the next two fighters will be an E3 reveal an the final character in the pass. E3 reveals are always mega ton fan favorites so I am fully expecting a 3rd party legacy character similar to Banjo and Steve. In this case, the possibilities are endless. Perhaps they can complete the Microsoft holy trinity by revealing Master Chief. Dante, Doom Slayer, Crash etc. I know its still a long ways off but I think this could be one of the biggest reveals in Ultimates cycle.
Assuming there is no 3rd Pass, I am also expecting the final character to be a pretty huge deal. Yes, I know they ended the 1st pass with Byleth (and we all know how well that went over), but from how things have been sounding it seems like Byleth was never truly intended to be the last character. I don't even know who they could end with, but I can assume it would be a major 3rd party. Something Crazy like a Sony Rep or something completely unexpected.
Safe to say, I will be very surprised if we get another 1st party character. With only two characters remaining, I want to see some big gaming icons be included. Only time will tell I suppose.