This would be based on the events of the manga:
Ichigo manages to rescue Orihime, after defeating the espada Ulquiorra Schiffer. But unable to find a way back to their world, Ichigo and the rest are left stranded on Huenco Mundo. Meanwhile, It looks like the soul reapers are barely winning, with the 'four elite soul reapers' protecting the pillars, The captains proceed to fight with Barrigan. Barrigan is defeated and soon, the other espada and Aizen, Kaname and Gin come out. Captain yamamato, (1st division captain) Remains passive. Gin engages with Izuru Kira, Who leaves his post and fights against him, to be defeated and rangiku takes over and wins. Kaname squares off against Saijin [It should be this way, pretty climatic], Saijin wins, but with a dead Kaname, sheds tears and vows to upkeep his beliefs. Aizen defeats the rest of the captains and soul reapers with ease, but not without having a long fight with Toshiro, who desires revenge for what Aizen did to Hinamori, leaving the espada that are with him and Him to fight General yamamoto. Yamamoto is almost at the losing end. Till a mysterious arrow hits the scene....
*scene changes to Huneco mundo*
As Ichigo and the rest, including Kuchiki Byakuya and Kenpachi sit around a certain area, waiting, out appears Urahara from nowhere whom gives them a way back to the real world to assist the battle. Ichigo pauses, seemingly unwillingly to help out. But then, Ichigo thinks about the friends that would be in trouble if Aizen was left undefeated, Ichigo enters the portal along with the rest, to protect his friends!
*back to scene*
Ryuken Ishida, in his quincy form, along with Isshin Kurosaki enter the scene. Aizen engages with Ryuken and Isshin, who prove to be a match for him. But just when the duo are almost seemingly to be defeated, a portal opens, with it comes out Ichigo and the rest. Kenpachi, Byakuya, Renji, chad, Orihime and Uryu engage with the other espadas that Aizen has brought. Meanwhile, Ichigo begins to duel one on one with Aizen. As the duel continues, it seems Ichigo has the upperhand when Aizen makes use of his shikai to defeat Ichigo. Ichigo is helpless before Aizen's shikai, leaving him all thrashed up. Just as Aizen is about to do the finishing blow, Shinji and the vizard step in. Kisuke then opens another portal which Isshin Kurosaki drags Ichigo in. There, Ichigo and Isshin have a small chat, both father and son coming to a understanding. Then Isshin tells Ichigo that if Ichigo can defeat him, he will have a chance against Aizen. Ichigo nods, and sqaures off against Isshin. Finally, Ichigo is almost at his wits end again when Zangetsu appears and tells Ichigo, to remember, that this time, the battle is to win, not for winning, but for his friends. Ichigo recalls his resolve when he became a shinigami and realises that he has been figghting to win battles to survive recently, unlike the other times like when he was trying to win battles to rescue Rukia. Ichigo takes up his Bankai again and defeats Isshin, his resolve pushing his reiatsu to further heights. Isshin gives a nod of approval to Ichigo and opens the way back to the place where Shinji and the other vizard are fighting Aizen, whom want him dead for what he did to them, but are losing.
Aizen grins as all the other vizard lie on the floor, defeated and wounded. Shinji is barely standing, trying his hardest to get up and fight. Aizen laughs at Shinj and then scoffs at him. As Aizen raises his blade, a portal opens, and a flash comes out of it and just in time to block Aizen's attack. Ichigo reappears and Aizen senses the difference in Ichigo. Ichigo and Aizen square off. Aizen uses his shikai, it hampers Ichigo, but very soon Ichigo overcomes Aizen's shikai. Aizen resorts to his Bankai. Ichigo and Aizen begin to clash and clash very hard. Reiatsu is being given out from their duel. Eeventually, Ichigo manages to even go further then Aizen's limit in the fighting style of zanpakto.Aizen is shocked as Ichigo gives him a finishing blow. Giving his last words Aizen asks, "How on earth.... did... you overcome the limit...?" Ichigo remakrs that it was his resolve that brought him this far. Aizen dies and things go back to normal.
The aftermath...
Ichigo and the rest go back to their normal lives, with Rukia and Renji along with the other soul reapers returning to soul society. Back in soul society, a year kater, General Yamamato is sent to the royal guards for his great service to soul society. Toshiro is appointed the 1st division captain due to his skill. Everybody's lives resume as normal. Rukia and Renji meet with the Shibas and remember the good times with Ichigo and the rest. Back in the real world, Ichigo and Orihime become an item. Everybody's lives resume as per normal. Ichigo recalls the first day he met Rukia and how his lives along with his schoolmates have changed. Then he senses a reiatsu of a hollow, gets out of his body and into his soul reaper form. Then the scene ends with ichigo dashing off, with these words as his thoughts, "I will protect my friends, till the very end... For I am Kurosaki Ichigo, Soul reaper substitue!"
End.
The ending's a bit off, because I am a bit hazy on some facts. But this would be my ending.