General Antonyms:
* Improvise: To create or do something without planning in advance.
* Spontaneously: Happening naturally without planning.
* Leave to chance: To allow something to happen without planning or control.
* Open to possibilities: To be receptive to different outcomes or options.
* Let happen: To allow something to occur without interfering.
Antonyms Related to Specific Meanings:
* Unplanned: Not planned or predetermined. (Focuses on the absence of planning)
* Unforeseen: Not expected or anticipated. (Focuses on the element of surprise)
* Unpredictable: Not able to be predicted. (Focuses on the lack of predictability)
* Flexible: Able to adapt or change according to circumstances. (Focuses on the ability to adjust)
* Contingent: Depending on something else for its existence or outcome. (Focuses on dependence on external factors)
The best antonym will depend on the specific context in which "predetermine" is used.