Requirements define the problem boundaries within which an engineering team tries to find acceptable solutions. Gaps in requirements formulation can lead to solutions that are not fitfor-purpose. However, the completeness of a set of requirements cannot be demonstrated; rather, completeness is an attempt, a best-effort pursuit. In current practice, where requirement gaps are frequent in system development, the human (engineer or team of engineers) becomes a major factor in the comprehensiveness of the resulting set of requirements. In this paper, we present a concept of an intelligent systems engineering (SE) advisor that supports the (human) engineer in identifying gaps as requirements are formulated, the set of structural rules that the intelligent SE advisor uses to perform the assessment, and a proof-of-concept implemented as a plugin for a Systems Modeling Language (SysML) software environment. The proposed intelligent SE advisor evaluates requirements that are formulated in the form of models leveraging a knowledge repository to read the model-based requirements. If potential gaps in the set of requirements are identified, these are presented to the engineer, who decides how to address the gaps. In this way, the intelligent SE advisor contributes to assessing requirements validation, beyond simply verifying model construction.