That would be the worst idea ever. In the whole history of console gaming, only two add-ons have ever been (moderately) successfull: Sony’s dual-analog controller for the PS (which was available pretty quickly and was then bundled with the console), and the memory expansion for the N64 (which was bundled with some games, I think) - at least I can’t think of any other successfull add-ons. For example, all CD drives for cartridge-based consoles failed. The reason is that the whole point of a console is to have unified hardware. You don’t want to split the people who own your console into two groups. You don’t want your customers to shell out money so they can buy games (since that’s where you make money). You don’t want to create games that only some of the Xbox owners can play.

Add-ons for consoles are a lose-lose-lose situation.

The HD-DVD add-on has never been about gaming. It’s always been about fighting Bluray on the movie front, bringing cheap HD-DVD movie playing capability to Xbox owners.