As I might have mentioned before on other pages, Nintendo, Sega, and NEC locked themselves into a "vaporware arms race" throughout the 80s and early 90s. It probably began with the add-on BASIC interpreter and keyboard that every console maker felt they had to release to turn their little systems into "computers." Sega did the same with their SG-1000 II, and NEC tried to do it with their PC-Engine Tsushin Booster much, much later (they never released it.)
Next up was the disk drive unit, which Nintendo infamously released, flogged, and kept on life support until 1992 or so. Sega felt the need to copy them and tried something similar for the Master System (never got released) and, as can be seen here, for the Mega Drive. Other small weapons in the arms race included a modem (the Famicom had at least two different ones) which Sega released only to face complete apathy, and which, again, NEC tried but failed to release; a drawing tablet, of which Sega was the first to release for their SG-1000, NEC second (a light-pen thingy), and finally the Famicom (Bandai in 1990.)
It must be said that this copying by various vendors was done in a typically Japanese style: long periods of no innovation as companies were too risk-averse (euphemism) to be the first to try something new; then when one company finally summons the courage to innovate, all others scramble to copy the innovator. It's a circular follow-the-leader. Later this race took on an anticompetitive tone as one company (usually Nintendo) would announce a distant new piece of hardware at the same time as a second company was beginning to sell theirs, thereby souring consumers' interest in the hardware that was just released (hello, SNES CD.)
Anyway, as can be seen here, Sega showed off in late 1988 a keyboard, drawing tablet, and (nonstandard 2-inch) disk drive (and a modem, too, of course) for eventual use with the Mega Drive. None of these pictured were ever released, though the existence of a working tablet has been confirmed. Would these have succeeded if they had been released? I really doubt it, considering the MD's Japanese user base of older arcade-lovers and shoot-em-up fans.