I'm not inclined to give credibility to someone because they predicted Ridley, by far the biggest fan request, and Isabelle with an Animal Crossing Switch announcement, the most logical marketing decision imaginable. The only leak that really deserves attention at this point is Vergeben due to Simon, but between missing K. Rool and his shockingly poor behavior this week in reaction to the Pokemon music track, I think even his word needs to be taken with a massive grain of salt at this point. Again, I'm looking forward to Minecon on Saturday because, even though I don't expect a reveal there, I think the day passing without at least a "We have exciting news from Nintendo coming soon" would be a very interesting development.
Quick thoughts on the other new pieces of speculation. Regarding the first DLC character being a Nintendo Online incentive: I think that's very plausible, but I have to believe they'd pick someone with more mainstream appeal than Geno. The Mewtwo promotion makes sense as a veteran fighter and a character that pretty much the entire gaming world is familar with; I don't think a Super Mario RPG character is remotely on the same level. As for stages, I don't think finding five new stages is a problem regardless of the exact number of newcomers remaining. As an example, I think Jon from GameXplain's idea of a Tetris stage is extremely plausible, and there's already been tons of speculation of Minecraft's representation being a stage. Finally, on the subject of Mute City's renaming, I think it's simply a clarity move, but I think it may also signal that they want to leave the door open for the remaining veteran stages to come back later as DLC. Regardless, it doesn't mean much at this point other than "This is the Mute City stage styled after the SNES F-Zero".