I'm just worried about what's going to happen to the huge Game Boy (line), and PSX/PS2 game libraries after NES and SNES fan translation are played out. In some ways I feel like the Final Fantasy series created this hobby by making a huge demand for the NES/SNES titles that weren't localized in the west. Who now is pining for those PlayStation games that were never localized when they were a kid? Game Boy/GBC? Game Boy Advance? PS2? Probably nobody, despite the enormous libraries and number of games that only came out in Japan.
GBA and DS is more or less my thing. No shortage of people interested in those before, during and after hacks became viable there, and after those consoles no longer kept having games (granted I have long held the 3ds library was pretty poor when all was said and done). Translations might be less common, or indeed not the default activity/synonym for a lot people when you say ROM hack and their interests are said consoles, but improvements and custom versions of a whole bunch of games is common as you like. All the tools are made (pretty much only FCEUX and PC debugging tools are better), the hardware well documented, many techniques known and documented, the libraries as a whole are known and explored well enough to know many common formats and methods employed...
I am more worried about the 3ds as part of all that actually, or not as 3ds has no games.
I am less versed in the GB/GBC so tend not to get involved in the really technical side of things there but it does not do too badly either. Indeed there are plenty of people seemingly getting into exploring it and realising there is some good stuff here and there are not so many annoyances when it comes to the hardware.
Not sure what to say about the PS1. It is all pretty scattered -- I occasionally go looking or otherwise stumble upon sites and find some interesting stuff, and people clearly playing with high end techniques to find how the games work (though the differences in terms says to me they are often pretty isolated* from each other). Similarly the legacy of no standards** for rips is going to loom very large for many years to come. epsxe's plugin nightmares probably did not help matters but there are some decent debuggers.
*I will spare you the sight of me going full internet communities anthropologist but it is fascinating. The best case for me is probably pokemon vs the various consoles it was on -- the techniques are often split off and developed in some odd ways, said techniques might be developed from earlier attempts at things and never refreshed so they are still doing very hard things where others might have automated tools and workarounds, the terms coined/used are often not those the rest of the ROM hacking world or console specific scene uses, tools are also often forked before being worked up for years and never refreshed from the originals or sent back to the world at large outside of people like me going and grabbing them, there are often perceived difficulty barriers (pokemon and assembly hacking showcasing all the things I have noted thus far, at the same time as having a light assembly hacking being a fairly standard progression in it). Or what passes for Japan's and China's ROM hacking scenes, though the language barrier and respective cultures make that more... reasonable, plausible, understandable... I need a better term but hopefully you can see where I am heading.
**The Scene was a thing back on the PS1 but I guess they did not have as much clout as they did for subsequent consoles, or during the likes of the apple II days. PS1 titles also employed various amounts of anti piracy and unofficially many of the competing CD ripping and handling tools (all naturally with their own format they were pushing) so you have loads of formats, and multiple versions of games, to say nothing of people wanting to use a normal rip they might have made themselves (which is probably very accurate but nothing like the legacy rip and something most patch formats will struggle to handle well).
PS2. The difficulties of ISO distribution (I don't know what a full set is for it these days but it is still enough that you are not going to see it, or sites featuring it, be that popular any time soon) probably hurt this a lot. That said the emulation is coming along nicely, and there are people doing good work too.
I would probably be more worried about the N64. Emulation is moving again but is pretty stagnant, the hardware is a pain, the game library is ageing badly by the year (I have a fairly decent N64 library as I chucked it all in a box and thought nobody would care in the future, that said I can't play most of them as the graphics and framerate make my eyes bleed. Give me said games redone on newer systems and I will probably have a whale of a time -- was going on the Rare replay for the xbone a while back and was good stuff, I have long held Perfect Dark XBLA to be a fantastic game that has things to teach the games of today with my main issue being I never quite figured out a way to map things so the aim and side strafe move that various levels were seemingly built around felt as good as it did on the N64 pad.
Original Xbox might also be a fun one. Between ISO sizes, rips and its own legacy issue (for simple DVD modded 360s then the ripped isos will not work so various people set about redumping things). Translation wise then as it flopped in Japan there is not a lot there really (though some things will need looking at) but plenty of people made nice trainers for just about every game, and format wise it can probably pull from the PC of the time/since and other formats.