Either way, it would be criminal if Walt didn’t make even a little appearance on the show that he helped spawn.
Like perhaps Gus passes him in the hallways of his school, since Gus was all over New Mexico, and Walt wouldn’t know him at that point, and would likely just pass him by.
If he were to make an appearance, it would obviously have to be in the main story line since he’s dead.
Well, it would pretty much only have to be a cameo, as he would still be a teacher at this point.