It's entirely possible that a united japan could end up conquering china, which had long been an ambition of theirs.
With china's abundance of land and natural resources, the one thing japan really lacks, they'd be quite a powerhouse.
China... is
really big, and throughout it's entire history has always been able to field absolutely massive armies. The
quality of those armies often left much to be desired, but trying to defeat them and then hold vast swaths of land with fuedal Japan's population would be an absolutely insane undertaking. The population difference was approximately 15 to 1 in China's favor, never mind the sheer amount of land involved.
In short, trying to end the Ming Dynasty a century ahead of schedule would probably take the rest of Shizuko's life, unless the author gave her some insane handwaves.
Which the author probably would, but still.