In this paper, we explore a new approach for a unified and interdisciplinary parametric model for estimating the total engineering effort in developing and delivering a software-intensive complex system. We begin by reviewing some of the limitations of using existing engineering discipline-focused tools for estimating total engineering cost and by articulating the benefits of such a holistic model. Applying a two step method combining heuristic analysis and data validation, we propose three hypotheses to expand the basic cost estimating relationship of COSYSMO, a systems engineering model, to the total engineering scope by including software size drivers. The implementation of the hypotheses and the validation approach are also discussed. We conclude the discussion by outlining the future work required to realize such a model and to apply it to supporting successful system development endeavours.