With all the craziness behind the Grinch leak that keeps piling up, the reactions from the insiders and their conflicting information, and the possibility of false info making the rounds, I'm really not sure what to believe. Only things I'm sure of are that Ken is in as an Echo, there's at least two more non-Echo newcomers, and that neither Banjo nor Geno would end up an AT.
If Isaac is an AT, it would be quite ****ty that Sakurai didn't reveal it sooner at the two prime opportunities to do so with the first two Directs and revealing it this late doesn't sound like the sensible thing at all. However back in Smash 4 Sakurai made a very subtle tease about Ridley's stage hazard status on Pyrosphere in an August 2013 POTD, then said nothing else about Ridley at all until 8 months later with the infamous shadow tease in the April Smash Direct, and then never said anything more about it up to the 3DS' version release, with Smash3DS not even having a Ridley trophy, until finally confirming that he was indeed a stage hazard on Pyrosphere in the 50 Facts Extravaganza. So considering the Ridley Smash 4 situation I certainly don't think it's infeasible for Isaac to get revealed as an AT this late in the game after the apparent tease tweet and possible Psyenergy item.
Whatever happens roster-wise I personally won't be affected as I already got all my five big realistic roster wants for the game (Wolf, Ridley, Belmont, K. Rool, and Snake); Mach Rider would be interesting to see how they would turn out and I think Incineroar has the potential to win me over depending on how he is implemented, however I'm not invested at all in them nor any of the other implicated characters making it in, though I would certainly like the Grinch leak to end up being true or a greater scenario occurring for the sake of others.