On "completion" then I do recall a few years back someone aspiring to get every podcast, this however was when things were winding down such that it was a bit like usenet text discussions, and before Joe Rogan and whatnot made it cool again and everybody and their mother decided to start one. I suspect something similar will happen here in that you will unlikely get to a notable level of completion, or will have to make some considerable curation efforts beyond what even this site has.
I do try to keep an eye out for whatever is popular in the game specific world. They tend to get tools and easy to follow guides which in turn sends the noise levels way high for the resulting hacks (there are only so many changed the starter pokemon hacks/remove all challenge hacks I care to see).
I don't know if it is top games that get top billing, and there are certain exceptions thanks to a few dedicated individuals. I do occasionally find Japanese groups doing good work, Chinese groups doing a lot, Brazil has a fair bit worth noting and occasionally Thailand or Vietnam will have something (both of those rarely get games but have seriously high computer literacy). Not seen much out of South Korea. Russia has some decent flash cart and emulation things (see something like gbx.ru) but less so on the ROM hacking front for specific games, has some really nice tools though ( http://www.magicteam.net/index.php?page=programs
Other than Japan most things I see will be translations though. Afraid I don't really have much in the way of current links as much of my bookmarked sites are not active, and the others usually have ROMs too. Never found a good umbrella site for Japanese ROM hacking (or emulation/flash carts and homebrew) but given how copyright is serious business even a lot of the Japanese public is (see people making fan art on Japanese twitter) I am not entirely surprised there.
Still Nintendo forms a fairly considerable portion of the ROM hacking specific games world
Mario in its various incarnations gets hacks aplenty.
Usually it will be the various platformers, though they might split by console generation and 3d vs 2d even then (the new super mario brothers crowd does not always overlap with the whatever 3d is going then stuff).
The more popular games also see the greater volume of hacks -- Mario 64 will tend to have more than Sunshine, and thanks to that source release has even more happening. not seen the most for the Galaxy games yet.
For the sake of linking a few DS efforts then https://nsmbhd.net/
has some stuff, mostly new super mario brothers but had a bit of Mario 64.
The Mario Kart games also get various things http://wiki.tockdom.com/wiki/Main_Page
A few people seem really into Mario vs Donkey Kong too.
Kirby and Wario don't get nothing but really not a lot when all is said and done, especially given how similar they are to a lot of other games that get all the hacks and they have considerable fanbases too.
Mario Party sees a few people try some things, though usually more in the cheat side of things. I am seeing some increased interest too.
Smash Brothers has a lot of quite advanced stuff, though often tied to competitive which means the gamecube and wii versions mostly.
Do a search for smash brothers mod packs and all sorts of things will come up.
Zelda does a bit. I would say 2d more than 3d but 3d has a fair bit, and usually most things in Zelda are more in the improvement world (I would not play the DS entries without the dpad hacks).
Pokemon is huge. https://bulbapedia.bulbagarden.net/wiki/Main_Page
is a wiki that most will know and https://www.pokecommunity.com/
is probably the main place people go, but there are endless others out there (often doing good work like https://www.dragonflycave.com/mechanics/gen-i-capturing
Its one time rival in Digimon has some things at times as well.
Fire Emblem has a fair bit of stuff, and a fair bit of drama.http://www.feshrine.net/ https://forums.serenesforest.net/
Golden Sun has a surprising amount of things, and its share of drama toohttp://forum.goldensunhacking.net/
Metroid rarely has a single site dedicated to it, does have enough dedicated hackers that churn out interesting things. This will mostly be on the 2d stuff (so NES, SNES and GBA) but 3d is having some inroads made.
Lots of good stuff there, don't really find much of it outside that site.
Its sometime competitors in Daisenryaku and Nobunaga's ambition have a bit in Japan and China, no real links I have for such things though. Nobunaga's ambition has more recently seen some spinoffs in things like pokemon so some have gone there, and pokemon spinoffs like pokemon mystery dungeon (itself an entry of sorts in the Mystery Dungeon series) attract attention.
If Advance Wars was not niche then the Fire Pro Wrestling community certainly will be. I don't know where they are hiding right now (usually see them on various free sites which makes keeping current harder than is ideal). However translations, modernisations, tweaks and more speaking to the heart of the games often happen here.
For the sake of having something then http://firepro.ewzzy.com/ffpw_2010.php
Rare efforts on the N64 have a bit of a following. Banjo Kazooie most of all (see something like Banjo's Backpack). I see less for Goldeneye and Perfect Dark than I might have otherwise expected to, though the glitching, challenge, speedrunning and tool assisted speedrunning communities have some nice info.
Castlevania of the 2d persuasion usually gets stuff on a routine basis (mostly SNES, GBA, DS and the occasional PCE/TG16 effort though the megadrive has some, gameboy has some interest and a few people are toying with the PS1 effort and XBLA effort when I go looking). Some masochists keep insisting the N64 efforts have redeeming qualities and doing things for them. Few people seem to be interested in the more recent Lords of Shadow spinoff/reimagining on the hacking front, though from pure gameplay they are thousands of times better than N64 even if I can quite happily accept a designation of Castlevania in name only)
Square/Enix/Square Enix games often have a bit of a following.
Final Fantasy is the big one. Most of the time it is translations and minor tweaks/bug fixes.
Final Fantasy tactics on the other hand. That often sees some fairly radical hacks that rebalance whole aspects of the games (PS1, GBA, DS and probably whatever newer stuff gets made).https://ffhacktics.com/smf/index.php
is a decent start.
Chrono Trigger (slightly less Chrono Cross) used to be popular but drama and some dubious legal notices changed that one.
Dragon Quest mainline does not get the most attention (despite being huge in Japan it does not make that much of a dent outside it). The spinoffs for slimes and caravans and whatnot are a different matter.
I occasionally stumble across fairly advanced information for a lot of other random Japanese RPGs (Suikoden, SaGa, Breath of Fire, Monster Hunter, *spits* The Legend of Dragoon, Xenogears and Xeno*, Tales of,... in random forums, gamefaqs discussions and fan forums for the games in question, how much that generates in the way of hacks as much as info on optimal play strategies is a different matter.http://ngplus.net/InsaneDifficultyArchive/www.insanedifficulty.com/
has some stuff that might be of interest, though it is now a dead site and I don't know where people there landed.
YuGiOh sees some determined people wander in every so often. Being a collectible card game ( https://magic.wizards.com/en/articles/archive/duels-planeswalkers-all-about-ai-2014-02-13
) they soon leave again when the difficulty becomes apparent. Still it, and some other CCGs, get the odd multiplayer focused hack.
By similar token the various anime franchises that spawn games see fans try to wander in and do stuff. This rarely amounts to much. Still an eye on Zoids, Dragon Ball ?, Hamtaro (seriously), Inazuma Eleven, and whatever is pointlessly popular but will not be remembered really in 5 years might have something.
Phoenix Wright kind of has something maybe, though usually more on the translation front. I have seen things designed to make cases, and some role playing type deals for it as well.
Sonic (mostly 2d, which is fine by me as Adventure is an abomination), Streets of Rage (though this is probably more focused on Beats of Rage engine these days), Shining Force (mostly the megadrive/genesis efforts rather than Saturn. Less so Shining Soul and Shining turn based strategy, but I could see either pop at some point), Golden Axe and Phantasy Star RPG (not so many doing stuff for online, though there are some types doing custom servers and translations) are probably the main ones getting attention here.
For Sonic there is https://sonicretro.org/
, and it should also be noted Sega came out the other year and said hacks are good, feel free to share them on Steam and such for a lot of its games
Someone usually picks up and runs with Zombies Ate my Neighbors every few years. From where I sit the level pack thing around here is probably one of the best examples of additional levels from fans that any ROM hack has ever produced.
Street Fighter has the odd hack, as does Mortal Kombat. They are usually more about unlocking things or making the ultimate version featuring everybody. Lesser versions of this also apply to other vaguely popular fighting game franchises (King of Fighters has some, Virtua fighter a few, Tekken a few, Marvel vs,
A few people do the Disney and Disney Capcom platformers on various systems. Not so much in the way of total conversions or serious tweaks though.
Equally if you are not following cheat making then you are potentially missing out on a lot.
Blurring a line a bit here but when Doom, Quake and all that get ports/remakes/whatever on new consoles they often come with various level packs, and people coming the other way and doing some considerable mods to them. This might dry up as most modern consoles and handhelds and phones can play 10 copies all networked on the same device and barely tweak the utilisation needle so we tend not to see so much in the way of creative porting efforts.
I had once hoped Bomberman would get more attention than it does, sadly not to be. Speedruns and glitches do well though, surprisingly including a lot of the 3d efforts (I like them but such opinions, along with my fondness for atomic bomberman, got me thrown out of the bomberman appreciation society).
Megaman often gets a lot of attention from a lot of hackers. This tends to be more on the translation and bugfix side of things. Whole bunch of sites this gets split between, though the big split is usually the platform vs RPG (battle network). Platform might also see a split between the mainline, X/Zero/ZX and Legends spinoffs.
Sports games in general often get a team roster update to make an old game compatible with current real life one, as well as maybe a rules update if a rule changes in a sport. Some might even have some hacks to change gameplay a bit.
Enough waffling for now though. I have probably forgotten a major franchise that sees all the hacks but I will leave it there for now. If you wanted to download example hacks here and check readmes, thanks and recursively search for the hacks in question to see where else they are listed then you might get some places to bookmark.