The core already has available: video > NTSC overscan + PAL overscan audio > buffer samples(maybe it’s by default in RetroArch) + lowpass cutoff Hz + Gain (dB): Overall + FM + PSG + FM DAC (no idea about this)
System: MegaDrive models + Region + 68000 Clock divider (no idea) + Initial RAM Value
I’d appreciate if it’s ever possible to allow those options in RetroArch any of those days.