"I don't suppose" is a common phrase used when making a statement that is unlikely or improbable. However, there are many synonyms that can be used to convey a similar sentiment. Some alternatives include "unlikely," "improbable," "doubtful," "uncertain," "questionable," "unimaginable," "unexpected" and "unforeseeable." These words indicate a lack of confidence in what is being stated or suggested. Using these synonyms adds variety to your language and can help to convey your thoughts in a more expressive way. It is important to choose the appropriate synonym to fit the context and tone of the statement being made.