LaserDisc add-on

Ahh, the legendary expansion port and the LaserDisc interface. Yes, the idea of connecting the 7800 to a LD player may have existed during the original design, I'm not sure how far it made it past the concept stage or whether it is, in fact, possible. 

The expansion port is almost entirely outputs, not inputs. So there would need to be an external video mixer which would combine the video from the LD and the 7800 (and sync the two somehow, not a trivial operation since the capability isn't built into the 7800 or the LD; maybe the EXTMEN signal was meant for this). There would also need to be some sort of interface from the 7800 to the LD to select specific frames (CAV tracks) via the joystick port.

But the big problem is the 7800's video is not standard NTSC (See Laser Disc Memo: http://www.atariage.com/forums/index.php?showtopic=72599) It's half a colorburst too wide and non-interlaced. Now, it might be possible to record an LD with matching frame characteristics, but you might end up with something requiring a custom LD player. (Or maybe the EXTOSC & OSCDIS signals were meant to address this.)