An important question to ask is why you need to present an ART portfolio for a DESIGN degree? Designers don't take an art test when applying to a studio, they take a design test, it involves writing. So the most important skills are creativity and communication.
What you're describing sounds like an art lead rather than a game designer. I'm an amateur myself, but what you're looking to do seems infeasible. Designers don't often do art. That's what art leads do with the art team. Art leads also communicate with the programmers to get the right engine features for their assets. Game…