The phrase "leave up in the air" means to leave something undecided or unresolved. There are several synonyms that can be used in place of this phrase, including "uncertain," "undetermined," "pending," "unsettled," and "in limbo." Each of these words conveys a sense of incompleteness or lack of resolution. For example, if someone is uncertain about their future plans, they may be said to have left their plans up in the air. If a decision is pending or unsettled, it means that it has not yet been finalized or resolved. Ultimately, using synonyms for "leave up in the air" can help to add variety and depth to your writing.