@mydriaze Oh, the games still go in the megadrive folder, the platform.cfg file just changes the display and scrapping settings.
Oh, this is the txt you'd need in the platform.cfg file if you create it:
megadrive_theme="genesis"
megadrive_platform="genesis"
pcengine_theme="tg16"
pcengine_platform="tg16"
Then after creating that file and putting it in /opt/retropie/configs/all/ you need to update the TG and genesis cores, and you're good to go.