Back in 1977, Paramount began production on a new live-action Star Trek series called Star Trek: Phase II. The idea was eventually scrapped in favor of making The Motion Picture. But Mike Minor had already completed concept art for a revamped interior for the Enterprise.
A few of the scripts for Phase II were reworked for The Next Generation and the Phase 2 series of fan films, but a lot of what Minor created for the original Phase II exists only in his concept art. Above is the Enterprise's bridge (which would be used for The Motion Picture). Below are the rec room, captain's quarters, engineering, and what looks like a cargo hold. (Warning: some aspects of these designs are very 70s).
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