Moonlight is an open-source client for NVIDIA’s GameStream protocol. It streams gameplay from a PC to another device on the network — a Raspberry Pi running RetroPie, in my case — so heavier emulation than the receiving device could ever run natively becomes playable on the living room TV.
That covers the gap RetroPie leaves on its own: PlayStation 2, GameCube, and Dreamcast emulation are all too much for a Pi to run directly, but stream fine from a proper PC over Moonlight. I first tried it running Dolphin over GameStream to an Apple TV for Mario Sunshine at 1080p60 — smooth, no noticeable latency, genuinely playable.