Hey Author-san, are you familiar with the anime My hero academia? If so, can you imagine not only mc lived hundreds of years further than sasuke, but she is also an underground hero with a powerful quirk? When she arrived in the Sengoku Era, she has to keep it all to herself until there will be a time she needs to use it in order to save someone who is in a life and death situation.
(this is optional if you know mha...my brainstorming won't keep my mind in peace to be honest)