The phrase "be doing" can be replaced with a variety of synonyms, depending on the context and tense. In the present tense, alternatives could include "currently engaged in," "actively pursuing," or "in the middle of." For past tense, possible synonyms include "was occupied with," "had been working on," or "had been engaged in." In the future tense, other options could include "will be involved in," "shall take part in," or "is planning to do." It's important to choose the appropriate synonym based on the intended meaning and tone of the sentence.