Portable, like the Sega Nomad ("Phobos").
Hitachi was already making a 20MHz 68HC000 (or 68456 with an integrated DSP, or even add a single SH-2, like those in the 32X and Saturn), and the Megadrive's venerable VDP was already designed and had pins for a larger 128KB VRAM. With twice the VRAM, the portable could manage some of the Saturn's display modes (at least 320x240 and 352x240, 256 colors out of 16.7 million). Upgraded 12MHz Z80, much faster DMA, 512KB of system RAM, and improved polygon graphics.
Building a much more powerful Nomad that could play the Genesis' still growing library of nearly 1,000 games, the larger 30-40 megabit carts could compete favorably against Jaguar, SNES, and PS1 launch titles. A $99 screen-battery pack could be inserted into the small $99 console to create a portable for the market. Without the screen-battery pack, the Phobos could be plugged into the wall and TV, with two ports on the bottom for 6-button Genesis controllers.
A similar, twin-68000 Genesis with more colors was proposed by SEGA of Japan, but SEGA of America's own Joe Miller thought that an add on would make more sense. When Hitachi couldn't up the clock rate of their RISC chip, SEGA of Japan decided on two of the RISC chips for the 32X and Saturn, making both systems difficult to develop for.
The Phobos would solve that problem easily, since it could run a wealth of Genesis games that had to be slowed down to lower frame rates because of the amount of action going on onscreen.