There were a number of peripherals that took output from the NES -- at the very least, Family BASIC's data recorder,
the 3D glasses, and ROB. I was thinking of adding a section to the wiki on this, but it'd be awkward without more information — does anyone have interface documentation or items I'm missing?
Don't forget the Miracle Piano, it receives MIDI data via controller port #1.
There's also the casette recorder. The funny thing is, if you go into the ROMs for Excite Bike and Wrecking Crew, you can trace the code used for loading and saving.
This was very close to being one of the only NES peripherals that used the extension port, I think. It's a shame we never actually got it, especially since they clearly had intentions of releasing it.
How is that clear? Maybe they just left it in to avoid the cost of making new ROM masks or to reuse existing ROM chips. Just curious what your theory is for that.
They did have some plans to release it at one point - see the AVS items on display at the Nintendo World Store - and I actually think it's quite telling that Mach Rider had the save/load functionality as well, and it ended up being first released in NES form by about a month. I'm pretty sure they originally wanted to release the cassette recorder and ended up not doing so because the whole AVS reception was quite lackluster. I think it's a combination of reusing mask ROMs and keeping their options open - had cassette data storage lasted several years longer than it actually did, there is that slim chance that the US might have seen the recorder.
Look at the number of "to cart" signals on the expansion port. Then look at the number of signals on the cable from the Family Computer Disk System's RAM adapter to the disk drive. I'm pretty sure the expansion port was originally intended for the FDS until Nintendo scrapped the North American counterpart of the FDS due to widespread illicit copying.
Thing is, though - I never saw an AVS version of the FDS hardware, even in old magazine pictures (such as the only existing shot that I know of the actual AVS console itself). Only the data recorder. I do concede, however, that there are ten extra EXP pins that do match up nicely with the FDS cable, so I would say that at least during the hardware design stages, they wanted to keep the possibility open. I think, had the AVS been fully released, Nintendo likely would have at least begun working on a version of the FDS. As you mention, however, illicit copying was rampant in Japan and Nintendo probably just wanted to avoid that whole issue over here (they were quite successful doing so in terms of the general public, IMO - I never encountered NES ROM images until well after the system's commercial life, and I would assume that a majority of USA NES fans never did either). I never even knew that there were such things as cart copiers until I saw an old ad in Game Players for the MGH (interestingly enough, in a later issue of GP, they apologized for running an ad for a cart copier, likely because Nintendo probably bitched at them for it). I digress, however.
As I was typing this post up, more and more I'm agreeing with you in everything except for actually designing a US version of the FDS, I don't know of any evidence that they actually got to that point. Perhaps such a US FDS would have the RAM cart integrated, attached to the CPU bus through the expansion port. Even with that, IMO the lack of connected audio input would have been a dealbreaker (that actually seems pretty shortsighted if they did intend to at least consider a US FDS). Of course, knowing Nintendo, they'd include an RCA 2M1F Y-adaptor in the box and instruct users to externally mix the two audio signals.
I'm pretty sure a US FDS would have seen a FDS System Cartridge which would connect to the Drive via the expansion port pins that route to the cartridge connector. It would have been able to mix audio just fine. No cable from the cartridge to the Disk Drive would have been needed.
I know piracy is cited as the issue for FDS not even being tried here but I would think that would have been much less of an issue in North America at the time. But if Japanese developers can't market the game also in Japan without piracy plus Nintendo's horrible licensing, it's no wonder it never happened.