Don't listen to leaks unless they're from people with proven track records, and even then don't take them as gospel. It's honestly easier to just predict what's going to happen than it is to parse through leaks and determine what's true or not.
I predicted a while ago that we'd get a Direct in August or September, and another in October or November, with each announcing two or three newcomers and one to three Echo Fighters. I was pretty right; we got one in August, with two newcomers, and three Echo Fighters. So that means we're probably getting another Direct in October with two or three newcomers and some more Echo Fighters.
My original predictions for characters after E3 were based on observations of how much support they had/how much of an impact they would leave, as well as the pattern of "blockbuster" characters like Mega Man, Ryu, Cloud, Bayonetta, Daisy, and Ridley. They were King K. Rool, Simon Belmont, Skull Kid, Geno, Banjo Kazooie, and Sora. I added Ashley to the list later when I realized how crazy people were for her. Remember, these newcomers are mostly coming from the ballot results in 2015. Elma, Spring Man, Octolings, Rex & Pyra? Very unlikely.
I got King K. Rool and Simon right, but Ashley was deconfirmed, which I think was a surprise to a lot of people. So that leaves Geno, Skull Kid, Banjo, and Sora. But I'm replacing my Sora guess with Isaac, because I just didn't know who Isaac was at all, let alone how much support he has overall, and Sora doesn't have as much support as I would've expected.
My money is currently on Skull Kid, Geno, and/or Isaac. Banjo Kazooie is probably going to be a DLC character (maybe with Master Chief too, depending on how crazy negotiations might get between Microsoft and Nintendo/Sakurai). We might get all three in one Direct, or one of them might be a DLC character.
The question is, how much support did Isaac get back in 2015, when the ballot was going on and the project plan for Ultimate was finished? Was it a long-standing request, like Geno, or was it something that cropped up mostly after 2015, like Waluigi? If the answer is the former, then I think you guys have a good shot.
Thought the bigger question is how much of an impact would his reveal have. How big was the Golden Sun franchise, compared to the other contenders? Are a few million people going to cheer or jump out of their seats, or is it just going to be a few thousand? The headliners for the previous two batches were Inkling, Ridley, Simon Belmont, and King K. Rool. If Isaac isn't that caliber, then that presents a problem. But if he is, then it's likely. I wouldn't know; I know nothing about the character or the series.
Dark Samus was honestly shocking. Even after the evidence of Samus's Dark Samus alt color being removed, I didn't fully believe it, because she's not a very big character. Sakurai said that both she and Chrom were highly requested, but there's no way at all that Dark Samus was as requested as Chrom. I'm still trying to figure that out. So anything is possible, but keep expectations reasonable and objective.