If its manufacture has stopped then various things to take note of and contemplate for the future.
As others have mentioned then stock will likely dwindle down and possibly be liquidated when shelf space gets to be more expensive than sales, or Nintendo demands it to dodge warranty issues (I don't know what the longest will be but it can be up in the years range they have to maintain it for, if however no new items are sold and everything is "second hand" then yeah that is one part dodged).
Hacks is a fun one. If everything new in a box is hackable that is a nice thing to have, not as nice as any fully updated console but still nice. You may see a new coder or two try a silly attack vector, release one they were sitting on, work up one they did not want to do only to have it patched a month later, or otherwise see things refined to be almost drag and drop.
Whether we see a nice sky3ds style drop in and go flash cart I don't know. I have been hoping we see a "just buy this" cart for the DS (it plays all the games, homebrew will work find, has cheats, has no timebomb and is reasonable quality) for some years now and have nothing. However we are seeing good stuff on the GBA, GB/GBC and other consoles so maybe.
Online services.
How long these will stick around for is unknown to me at this point, and I am not sure I even have a good basis for speculation. The DS and Wii stuff was mostly stopped because gamespy was stopped, though I would not have given it too much longer at that point even without that.
It does cost money to keep it up, though how much I don't know, and as online shops are a thing there is also that to keep bringing money in*. Also there will probably be a law somewhere that forces them to do some kind of update (see the various consoles that got an update the other year when that EU privacy law came into effect) and that can be fun to try to push one.
*this might vary by country. It will either be the poorer countries last out by some margin (say it is popular there vs the Switch and they might as well keep that trickle coming in) or they will be first as the hassle of dealing with them is not worth the cost.
Games being delisted as rights agreements lapse, or companies no longer want to support a title, or companies get bought and sold, or sequels are going to come out so people don't want a bit of competition from themselves, will also be a thing.
I saw some mention a while back that Nintendo uses the 3ds as a gatekeeper for the Switch (why we still saw the odd game crop up for it) so new devs can prove they can hang on the 3ds and then get given Switch access. How long they can plausibly keep it up I don't know.
Spare parts, if they were not already limited, are going to get more so. I doubt Nintendo has any great stocks of things and probably has not updated them in any real capacity for a while (some places will do say 5% of boards go to spares and repairs department, however if Nintendo was not already winding down/ensuring some comfortable minimum then yeah).
I would not worry about batteries so much -- the way some of the battery tech is developing we will likely see custom drop in replacements before too terribly long for all manner of devices, or things so small but so potent that it does not matter if there is wasted space.
Screens, shells (maybe) and motherboards is a different matter and will likely be cannibalised or see Nintendo offer real money repair options. Whether we see a nice diagram of things made to do component level diagnostics and repair is up in the air, and more likely will happen or won't depending upon whether a handful of people put some effort in (and possibly sacrifice a few boards to map it out, assuming Nintendo's documents did not leak).
Homebrew. I don't look favourably upon the 3ds homebrew scene compared to the GBA, xbox, DS, PSP and Wii. It basically never got started from where I sit -- raspberry pi, android and IOS was doing a roaring trade by this point and the DS and PSP scenes had evaporated almost overnight when IOS and then android rose up.
Generally it will slow down. You might get the odd tech demo (
http://www.pouet.net/prodlist.php ). Some student at some point will likely do something silly for a final project just because (
https://web.archive.org/web/2012062...inerziegler.de/GBA/gba.htm#great GBA hardware ). There might be a bored coder or two that bumps out an update for something every few months, or ports something across to an existing engine, but unless something crazy gets discovered (some aspect of the hardware becomes a poor man's whatever or it interfaces super well with this year's hot toy, other than the 3d screen the 3ds is nothing special -- just an ARM device not so very different to the billion phones and tablets out there) you will likely see nothing too terribly interesting.
Outside chance someone does freeshop the next generation, though as things will be from their own CDN most likely then it will probably be a more limited affair.
ROM hacking and translations.
Will probably pick up pace on the translations front a bit, though I am not sure what is out there as far as big juicy targets for a translation (third rate RPGs tending not to get looked at until people are really bored) -- the 3ds has both a less than stellar library and left in Japan is not such a thing even if it was downloadable game format outside it. Whether one of the various countries that are not English speaking picks it up (I see fantastic work coming out of Thailand, Brazil, various Arabic speaking countries, China, Vietnam to an extent, Russia and there are also a handful of places in Europe that do things) is a different matter and might depend upon how popular it is there (though sometimes you get some dedicated translation groups). Hacks in general will vary even more as bored ROM hacker is more common, or at least most likely, and often way more prolific that bored coder. What we see there I don't know -- could be someone actually makes the 3d useful, could be someone improves controls, could be someone adds loads of levels...
Same for cheats, though usually there is is "we made a cheat for every game on the system, whoo" type deal than a fully thought through cheat database.
All this said most of this the same for every console.
