Isaac is in. I say this with 100% confidence. If he wasn't going to be in, Sakurai would have deconfirmed him with Waluigi, Ashley and Shovel Knight long ago. He wouldn't lead people on just to maliciously troll them. And the Picture tease? There's no reason to do that if you aren't going to add him. If he DOESN'T add him at this point the negative PR for the game will be insane.
Smash already generates plenty of salt on its own, no need to intentionally generate more.
I don't believe you, Isaac is 100% not in the game.
The fact that he wasn't shown whit the other popular assist trophies,
The fact that Golden Sun Dark Dawn website got updated twice,
The Fact that GS got new trademark,
The fact that there is only one new Nintendo LP in Ultimate
The fact that Sakurai decided to Edit a picture reminescent of GS on twitter, which is largely consider to be a GS reference and not a MH one.
The Fact that it got the exact same angle as GS battle view, which is super rare,
The Fact that's it's not the first time that Sakurai tease a character,
The fact that Sakurai is super carefull whit what he post because he hates to dissapoints the fans,
And of course Link=Isaac, silent protagonist,
Marth = Garet, one of the translation is Mars and he comes from Fire Emblem,
Shulk = Ivan because both can have the ability to see the future and posses the Shaman Rod/Manado which the bad guys wants.
Which are 3 RPG characters,
Against a fire dragon that can poison people like the fusion dragon,
And all of this in an Arena, whit an edit showing the assist trophy as if it was important,
And there is no one from brawl, were's the fourth guy?
Clearly guys, only an Idiot would believe that Isaac is in the game, it's only one super duper huge coincidence.
(Sarcasm)
I would rethink my life choices if Isaac is missing at this point lol.