Another idea: Tatsuki gets really, really pissed at Ichigo over the way he treated her prior to the Hueco Mundo excursion.
She gets visited by a demon, who offers her power. She accepts, and someone from the nakama sees her going through the gates to Hell, and thinks she's gotten kidnapped. So, the Nakama gears up, goes to Urahara, and promptly attack Hell in an attempt at rescuing her.
Ishida almost certainly winds up fighting deceased Quincy, who are not at all happy with him working alongside Shinigami. If Hell is anything like Soul Society, they'll probably wind up fighting a bunch of the more powerful Hollows who have worked their way up the ranks of Hell thanks to their initially more powerful spiritual power (compared to your run-of-the-mill damned soul).
The arc climaxes, of course, with a battle between Ichigo and Tatsuki, who has awoken all that power she's absorbed over the decade she's been Ichigo's freind, and become easily Captain-level, along with whatever demonic powers the demons gave her.
It ends with the demons bitching to the King about the Shinigami breaking their agreement, and threatening war unless they do something about the renegades who attacked them.