Huh. How about at a young age, after his mother's death, Ichigo imprints upon Trigun? In class, or the general public, he is the "class clown" and flirts while never actually intending to carry through on it. When a bully or other disruptive influences are around, he reverts to a more canonical Ichigo and deals with them as expected. Fortunately Tatsuki kicked any hope of shouting "Love & Peace" out of him, literally. Though what else would change around Ichigo, due to this imprint?
For Zangetsu and Hichigo, they would probably be replaced by Vash the Stampede and Millions Knives, respectively while the shape of Zangetsu would change into a certain .45 Long Colt Eagle, but shooting hardened Retsu rather then bullets. They don't explode, but they can be made to pierce tougher materials with more Retsu if/when Ichigo focuses enough. So fighting wise, he would go Hand-to-Hand while using "Vash" at range. Vash would give his standard speeches and lines, to the point of driving Ichigo crazy once he manifests, and be all for saving Hollows from their current fate. Though Vash would be utterly against how the Quincy deals with Hollows and...well I'll leave what he does to the poor Ishida later. Knives on the other hand, would be less training/instinct that Hichigo has going. Instead, he would know he is "superior" to Hollows, Shinigami, Vizard, and Arrancar as he doesn't fall into any of those classifications. He would be the corrupting force that would attempt to take over what Aizen is doing and lead it himself to genocide. Vash would be the one who teaches him how to dodge, predict, sharp-shoot, and fight hand-to-hand better but encouraging him not to kill. Knives on the other hand would "show" him the more destructive techniques, especially for Bankai, that range from wide area devastation to even soul destruction. After both awakening, they act as Shoulder Angels whispering into opposing ears trying to get him to do certain acts and driving him slightly bonkers. At least, when Knives isn't trying to fight him for control directly when Ichigo is weak. Though Ichigo does realize he life could be so much easier if he did follow Knives's advice and actually shoot a few annoying people. Maybe even repeatedly. Though that bounty of 60,000 yen from the Yakuza is getting annoying. He has been fortunate that the police agreed the collateral damage was at the fault of the people trying to collect, and it is nice that store owners will push a side a customer to ring up his purchases to get him out of there store fast, but the idiots just keep coming and recking everything around him. At least it is the stupid members of the Yakuza trying to take him down, as the upper levels are having a good laugh at what is happening, and charging "protection" to shops in the form that their Yakuza will not fight Ichigo in their shop.
The initial state would still be the giant sword, just to make things fall right into rescuing Rukia. His perma-Shikai though would be the pistol and a bracer/glove on his left arm. A bracer that forms a sub-machine gun into his off-hand when needed.
For Ichigo's friends and the effect of his aura on them? Orohime with Millie Thompson's giant Stun Gun that can and does launch her Shun Shun Reika as well as the standard ammo Millie uses. To note, the Shun Shun Reika are scared somewhat scared of being launched out of the Stun Gun that way. Orohime would still have her Rejection powers, just Ichigo's influence altered the style a tad. Orohime blushes still when Ichigo hits on her, but can never answer or accept before Tatsuki shows up and pulls her away while yelling at how indecent Ichigo is being.
For Tatsuki, instead of Long-cat/fox as a Zanpakuto and being cast a side early in the story, she would end up the equivalent of Meryl Strife. A cloak filled with basically infinite one-shot derringer pistols for a Shikai. As for Ichigo's imprint, she dislikes it and tries to draw him out of it repeatedly from taking and hiding the manga and anime to berating him for acting in such a fashion. As she never read or watched Trigun herself, she doesn't realize just how accurately she is portraying Meryl Strife until after her Shikai comes around. Though protecting Orohime's innocence is a bit more of a hassle for her this time around. From fending off Chizuru's normal advances, is breaking up Ichigo's flirting with Orohime. Tatsuki knows that Ichigo wouldn't normally go through with it, but Orohime might not let him have the choice if she finally gets fed up enough to go for it. There is no jealousy on Tatsuki's side to breaking up the two, none what so ever. It isn't like she wanted to have the "player" flirt with her ever...the @#%#$.
For Ishida, things for him would go normally until his challenge, the hollow bait, and the Menos Grande that showed up, and he tried to absorb Ichigo's retsu to power a strong shot. Unfortunately for him, Vash really did not like his Quincy Cross destroying the souls of he attacked, so he altered Ishida's Quincy Cross. Poor Ishida had a near break down at trying to figure out what happened to his precious bow, and why did the thing act like a Zanpakuto! Nor did it help that instead of a bow, the thing changed into Wolfwood's Cross Punisher. He doesn't have a Bankai, fortunately for his Sanity, but the activation to turn his Quincy Cross into the new style of weapon is disturbingly like a Shikai. "Heavy the burden, Punisher".
As the changes for Chad, especially since we are out of good guys for ideas? Instead of the Right Arm of the Giant and Left Arm of the Devil, why not have him form a black Saxophone with bone white highlights at first? Yes, essentially the attacks of Midvalley the Hornfreak at first. For his power-up in Hueco Muendo, he would get the "Arm of the Giant" on both of his arms for defense representing his grandfather while the Sax turns bone white with red highlights and becomes called "Tempting Saxophone of the Devil". Can't help but think of a Blues playing Chad for some reason.
Finally, for Ichigo's Bankai? Manga style Angel Arm for cannons and defensive feathers, Vash's signature Red Duster and sunglasses, and his orange hair turning black until showing how much power is left for his Bankai at first. Possibly a reflex boost as well.
I almost feel sorry for Soul Society with this setup. A running fight between Ichigo and Zaraki, where Ichigo's bullets shatter against Zaraki at first and Zaraki is destroying various buildings trying to crush the annoying bug. Angel Arm cannon activation against Byakuya will probably leave a lot of Soul Society in ruins, if not a crater.
