This idea will be relatively short, because I'm honestly not aware of most of the details of TYBW, or at least can't currently remember them, and thus I don't want to get into the details too much.
Basically, similar to Kisuke's plan for Aizen, I want him to have a plan for Yhwach in this fan fiction. He was mostly improvising with Aizen, but I don't want that to be the case here; I want a plan for Yhwach to have been thoroughly constructed, and to not have that plan be seen through by him.
This means less people will die, and that certain events in general won't play out the same way, if they play out at all. You can either have it so that Kisuke kept his plans for Aizen and Yhwach separate, or you can have it so that it was all one combined plan to have Ichigo beat them both. That's all I am going to say on this idea; the rest I'm sure you can improvise on with your own creativity rather than mine.