There are still a few games that do not run on the Mega SG because the mapper is not supported. They are listed in the Unofficial Analogue Mega Sg Jailbreak Supplement. I would appreciate if support could be added in a future release of the firmware.
Some of these games are already supported on MiSTer FPGA. Maybe parts of the MiSTer source, which is open source, could be used to improve the Mega Sg firmware.
For example the Taiwanese memory mapper for SG-1000 games is supported on MiSTer's ColecoVision core (which also runs SG-1000 games) and the Korean Mapper for MSX ports or MSX dual compatible games is partly supported on the SMS core for MiSTer.
SK-1100 keyboard support for SG-1000 is implemented on the ColecoVision core for MiST (which also runs SG-1000 games). The source can also freely be used as open source. On the Mega Sg, a XB∀ND keyboard could be used instead of the SK-1100 keyboard.
Also it would be nice if there would be an option to change the video palette in VDP video mode 2 (which is shared by SMS and SG-1000 games). Currently only the video palette from the SG-1000 VDP is available which is slightly different from the SMS' video palette. Therefore SMS games with video mode 2 like F-16 Fighting Falcon are incorrectly played back using the SG-1000 palette while they should be using the SMS' palette.