Man I can't stop dreaming about this! An example:
Full party:
Wave 1: 6 imps, 2 sprites
Wave 2: 3 lamiae
Wave 3: 4 spriggans, 2 ziz, 1 treant
Wave 4: 3 void sent, 1 Succubus, 5 yarzon
Etc, etc. mobs obviously designed for this and not just pulled from open world gameplay.