There are plenty of different synonyms for the phrase "could do," which can help add variety and nuance to your writing. Some common replacement options include "may do," "should do," "would do," and "might do." These phrases all convey a similar idea of potential or possibility, while also suggesting varying degrees of obligation or likelihood. Other potential synonyms for "could do" include "can do," "able to do," "have the capacity to do," "in a position to do," and "capable of doing." Each of these phrases may be more or less appropriate depending on the context and tone of your writing.