Some systems that have already been ported to the MiST/MiSTer share hardware components with other systems where a core doesn't exist yet. I have created a short overview:CPU
Z80A: Amstrad CPC, ColecoVision, Coleco Adam, Memotech MTX, MSX, SEGA Game Gear, Genesis, Master System, SC-3000, SG-1000, TI-99/4A
, ZX SpectrumGraphics
TMS9928A: ColecoVision, Coleco Adam, Memotech MTX, MSX, SEGA SC-3000, SG-1000 (modified versions used in the Game Gear, Genesis and Master System), TI-99/4ASound
AY-3-8910: Amstrad CPC, Intellivision, MSX, Vectrex, ZX Spectrum
SN76489: ColecoVision, Coleco Adam, Memotech MTX, SEGA Game Gear, Genesis, Master System, SC-3000, SG-1000, TI-99/4A (the soundchip used in the TI is identical according to Wikipedia)
Memotech MTX core for Altera DE1 (Cyclone II FPGA) with VHDL source: http://www.nyangau.org/rememotech/rememotech.htm
Maybe this knowledge can be used to improve some of the existing cores (by always using the best imlementation of one of the aforementioned components) or to create new cores. If the MSX core had a custom mapper for the SN76489 soundchip (like the Musical Memory Mapper) the SG-1000 games which can already be played using SG2MSX would also have sound.