Answer:
'Won't be'
Explanation:
'Won't be' would be the closest thing to future tense for the term not be. 'Not be' is more towards a future tense in itself, but if you are looking for something else, You would use 'Won't Be' as it works best as a future tense.
Here are some example sentences:
Won't be : You won't be a doctor.
Not be : You will not be a doctor.
