Yeah I'm going to go ahead and agree with Kendoki on this one, her power, the so called 'event rejection' is something that is probably near impossible to upgrade from.
To use Aizen's words
"Her ability allows her to choose a target and subsequently limit, reject, and deny that it ever happened, no matter what happened she can return the target to a previous state"
"She can easily supercede any event god has designated to occur ont the ground level. This power violates the realm of god."
You can't really upgrade an ability like that unless it becomes pure time reversal.
Maybe her upgrade wouldn't necessarily be something like Chad's or Ichigo's upgrades. It could be something like being able to do a better job with her current powers. For instance, maybe she can completely heal someone in two seconds or something. Or her shield could reject even a Cero attack at close range. And Tsubaki could actually do something. That little bastard can't seem to do anything.