Well, modern consoles _could_ still be extensible; but their hardware is already so general purpose that there's not much point.
Best you could do w/ current gen technolgy is bundle a dongle w/ the game, where the user plugs in some kind of co-processor through USB.
So far I haven't really seen anything like that -- the only USB dongles I've seen bundled w/ games are for games like RockBand and they're just RF receivers.
Aside from bandwidth concerns, and the poor sales of previous attempts (for e.g the SEGA's whole 32x/CD addon), there's nothing preventing a disc-based from having an external co-processor.
Best you could do w/ current gen technolgy is bundle a dongle w/ the game, where the user plugs in some kind of co-processor through USB.
So far I haven't really seen anything like that -- the only USB dongles I've seen bundled w/ games are for games like RockBand and they're just RF receivers.
Aside from bandwidth concerns, and the poor sales of previous attempts (for e.g the SEGA's whole 32x/CD addon), there's nothing preventing a disc-based from having an external co-processor.