A sure way for a video game console manufacturer to lose money and gamer loyalty.
The only expansion module that anyone ever bought was the ColecoVision's Atari 2600 adaptor. This thing fit into the front of the ColecoVision and allowed you to play your Atari carts.
It was successful, in part, because everyone already had Atari carts, Coleco didn't have to convince developers to adopt it. If only Sega realized that.