Basically, the divergence from canon is that when Ishida started the hollow-killing contest, a hollow that didn't come along in canon came along. This hollow was a hollow that was beginning to turn into a Menos Grande, after if figured out that other hollows tasted so much better. So, whenever it came across a soul, it ripped at their heart to turn them into a hollow for it to eat. It even managed to hollowfy and eat a few shinigami it ran across, though it had to adapt its hollowfication techniques to do so. It felt the numbers of hollows turning up to the contest, so it shows up and starts feasting on them.
Ichigo runs across it, and it senses how much spiritual energy Ichigo has, so it promptly attempts to hollowfy him, and it's surprised when Ichigo doesn't disperse, reform, and then grow a mask. Instead, he starts spewing his hollow mask first. He then promptly kills smashes the mask and kills it. The main effect of this is that Ichigo gets his instinctual knowledge of how to use his shinigami powers earlier (as this likely comes from his Hollow side), and starts his growth earlier. This may well affect the Menos Grande encounter later.
So, when Renji comes to get Rukia and starts spouting off about how Ichigo is 2,000 years to early to defeat him because he doesn't know his Zanpakuto's name, and starts pwning him with his shikai, Ichigo has a moment like he does when he's fighting Zaraki, and has a small conversation with Zangetsu, culminating in him releasing his shikai.
He might or might not be capable of forcing a draw against Byakuya. He could hold his own against Kenpachi with just Shikai, and Byakuya's at 1/5th power. If he can't force Byakuya to decide to retreat, he'd probably get pwned again, and lose his borrowed Shinigami power. This would be bad for Byakuya, because Ichigo would then proceed to go full hollow on him without the shinigami power to hold it back, and unleash his full Captain-level power, while Byakuya's limited to 1/5th of his total power. Then again, his hollow mask might appear inside his shirt, and deflect the blade just enough to miss his source of power. Simply getting stabbed through the chest isn't going to stop him, though, as the fight with Kenpachi proved, so the fight would drag on.
Regardless, though, Ichigo pulls out enough power to either force Byakuya to decide to retreat and gather more forces, or outright defeats him. How Aizen's plan changes to take this into account is up in the air. If Ichigo does go full hollow, though, he'd almost certainly be capable of retreating into using his latent shinigami powers to rip his mask off and become a natural Arrancar. Ichigo being recorded as having gone hollow might well result in him being approached by Urahara, who'd offer the opportunity to take Rukia and flee to Hueco Mundo. The rest of the nakama, of course, follows them there. Quite possibly including Tatsuki, if Orihime feels bad enough about abandoning her on a much more permanent basis to tell her what's going on.
While in Hueco Mundo, they, of course, run across Nel and her Fraccion. This might result in Orihime healing Ichigo's mask and/or training scenes.
Ichigo runs across it, and it senses how much spiritual energy Ichigo has, so it promptly attempts to hollowfy him, and it's surprised when Ichigo doesn't disperse, reform, and then grow a mask. Instead, he starts spewing his hollow mask first. He then promptly kills smashes the mask and kills it. The main effect of this is that Ichigo gets his instinctual knowledge of how to use his shinigami powers earlier (as this likely comes from his Hollow side), and starts his growth earlier. This may well affect the Menos Grande encounter later.
So, when Renji comes to get Rukia and starts spouting off about how Ichigo is 2,000 years to early to defeat him because he doesn't know his Zanpakuto's name, and starts pwning him with his shikai, Ichigo has a moment like he does when he's fighting Zaraki, and has a small conversation with Zangetsu, culminating in him releasing his shikai.
He might or might not be capable of forcing a draw against Byakuya. He could hold his own against Kenpachi with just Shikai, and Byakuya's at 1/5th power. If he can't force Byakuya to decide to retreat, he'd probably get pwned again, and lose his borrowed Shinigami power. This would be bad for Byakuya, because Ichigo would then proceed to go full hollow on him without the shinigami power to hold it back, and unleash his full Captain-level power, while Byakuya's limited to 1/5th of his total power. Then again, his hollow mask might appear inside his shirt, and deflect the blade just enough to miss his source of power. Simply getting stabbed through the chest isn't going to stop him, though, as the fight with Kenpachi proved, so the fight would drag on.
Regardless, though, Ichigo pulls out enough power to either force Byakuya to decide to retreat and gather more forces, or outright defeats him. How Aizen's plan changes to take this into account is up in the air. If Ichigo does go full hollow, though, he'd almost certainly be capable of retreating into using his latent shinigami powers to rip his mask off and become a natural Arrancar. Ichigo being recorded as having gone hollow might well result in him being approached by Urahara, who'd offer the opportunity to take Rukia and flee to Hueco Mundo. The rest of the nakama, of course, follows them there. Quite possibly including Tatsuki, if Orihime feels bad enough about abandoning her on a much more permanent basis to tell her what's going on.
While in Hueco Mundo, they, of course, run across Nel and her Fraccion. This might result in Orihime healing Ichigo's mask and/or training scenes.