The antonym of definite plans is indecisive, uncertain or tentative plans. Indecisive plans refer to situations where one is unable to make clear plans or decisions. Tentative plans, on the other hand, suggest that a person has plans but is not completely committed or sure about them. This could be because of external factors such as unforeseen circumstances or simply a lack of confidence in one's ability to execute the plans. Uncertain plans are those which are not clearly established and lack clarity or direction. Overall, antonyms for definite plans connote vagueness or lack of clarity, which can lead to unproductivity or wasted effort.