Final answer:
The missing word to complete the sentence is 'concept'.
Step-by-step explanation:
The student is looking for a word to complete the sentence 'Right now the idea was just an _______, and he needed to add details to the plan to make it more concrete.' Based on the provided context and suggestions for revision, the missing word should describe an incomplete or vague idea that requires further development. One suitable word to complete the sentence is 'concept' - an initial idea or plan that needs further refinement and elaboration to become more concrete and detailed.