What make a ROM hack very nice?

The question is simply.
What make a ROM hack very nice?
More pokémon, more moves, bigger map, better details, good upgrades?

Just like cooking, you can make better food with good ingredients.

But unlike the ROM hack is you can't eat it. LOL 😆
But seriously, what need to make this ROM hack very nice?

More pokémon, more moves, bigger map, better details, good upgrades?

Click to expand...

Big picture? These ultimately do not matter for capturing a certain feeling. I've played rom hacks that do all these things and were still hot garbage that were eventually lost to the internet. The truth is that anyone can add more "stuff" to a Pokémon game, so that's not necessarily something that makes it have that "very nice" feeling.

I think the feeling you seek requires a focus on three important aspects:

Polishing - Don't let your hack be a glitch-ridden mess. Do proper QA testing. Learn how to use a good debugger. Identify the cause of problems and fix them in a timely manner. Understand the root causes of bugs, implement proper fixes, and try to avoid bandaids and workarounds.

Attention to detail - Do your changes make sense within the setting? If you have an NPC for a new feature, why is that NPC standing there? Does it make sense for a water Pokémon to be encountered in the grass? Does that imply it's amphibious. Pay attention to why things are the way they are.

Tonal consistency - if the dialogue is mostly like a vanilla Pokémon game (childlike and whimsical), then don't have a character drop random curse words. Don't include modern internet memes if the game is set in the 90's. Likewise, don't have inane childish stuff in an edgey game with violence and crime. If you want to go for these things then you have to adjust the overall tone of the game to match. Keep the tone consistent so your players don't get mental whiplash.

so far the most enjoyable experiences I've had were those with plot consistency: wild Pokemon areas that have sense and purpose, background info from various NPCs, etc. One of my favourite romhacks so far was Pokemon: Normal Version, since it has great visuals and story.

On a technical pov I don't mind so much the phys/split, new moves, new pokemon, as long as I'm informed on what goes on. It's always nice when the changes applied come with available information.

Lastly, speaking about difficulty, I am not too much into difficulty romhacks, more of a casual player, so any quality of life enhancement is appreciated, for example Pokemon Altered gives a Lucky Egg being held by the starter, so it's a bit faster to earn exp. I remember Pokemon Flora Sky had some EV-training in the end of the game. But I may confuse it with Pokemon Glazed, I was playing both of them long time ago.

I think it revolves around fulfilling the player's "Pokemon goals". When someone picks up a hack, they're looking for a certain experience. I think at the core, those are:
1) Building a Pokemon team from a diverse pool of Pokemon (including searching for those Pokemon)
2) Raising Pokemon to be strong -- the feeling of progress (people want longer releases!)
3) A new story, as to not feel like they're replaying a base game

With these points in mind, you can further enhance the experience with:
- QOL changes (makes the required grind repeatable)
- Features which assist with experience goals (i.e Dexnav)

As such, hacks that prevent, or hinder the above will not be successful. Such qualities include:
- Bugs which break immersion, gameplay or cause inconvenience
- Hacks which look and feel like the default game (point #3), could be from maps, music, graphics, to scripts even.
- Hacks with intrusive story lines which hinder goals #1 and #2. Culprits can be excessive dialog, mandatory brickwall-esque battles or even frustrating puzzles in succession.

Things I think people mistakenly think a good hack requires:
- Good gym puzzles (this is something the developer wants. The player is happy with solvable, non-repetitive puzzles)
- Difficulty (Players who want this impose it upon themselves with Nuzlocke and randomizer runs). Unbound did a great job of making this togglable for players who were looking for it.
- Unrelated systems (crafting [i.e DPPT's Poffins], berry growing, ect. very few players care for this. You want to tie your systems into achieving the fundamental goals without trivializing said goals i.e overpowered exp gain)

I enjoy a ROM hack when it focuses more on exploration and the journey. Environmental story telling is more important than plot to the face. Keep those text boxes to a minimum because I don't want to waste my time clicking them away.

Technical feature: I can life without some improvements if I know what I'm up against. No phys/spec split? Fine, as long as I still have options to use "sub par" Pokemon in a meaningful way.

I like environmental puzzles. This is something that people still seem to have problems with implementing, however. As an example: If you exchange HMs with items you're just hiding a bigger problem here.

I really like when hack creators buff weaker Pokémon or give Pokémon better moveset access et cetera (especially when it isn't a difficulty hack!). One of my biggest pet peeves within the main series is not being able to use a Pokémon because it's just not fun to play with.

Other than that, I like having ubiquitous quests to do along the journey to break up the monotony.

When features are more modernized, like the P/S split, Fairy Type, and infinite TMs.

It varies from player to player, as not everyone has the same priorities or prefferences. People look for different things, including but not limited to:

-Abundance of features and things to stand out from the vanilla Pokémon experience.
-Updated mechanics, items, Pokémon from later gens, move pools, abilities, etc.
-New region to discover (More exciting than re-visiting Kanto or Johto for the 200th time)
-Huge Pokédex and being able to catch everything.
-Difficulty (some get bored if the game is cool but it's a walk in the park)
-QoL features (especially if the game is hard you have to provide the tools so in overcoming the challenge the game doesn't become a torture to play through)
-Suitability for Nuzlockes or other custom challenges.
-Post game stuff like battle tower and things like that.
-Edits/buffs to some Pokémon, custom Megas or new forms, and changes to their move pools, abilities, etc. for balance or for whatever reason (some like this, some don't)

And more. There's no formula that would please everyone. You may have a game with tons of Pokémon, huge length and bloated with features, but that game may be a grindy pain to play through because of absurd difficulty spikes or poorly handled game progression. Or you may have a game that perhaps doesn't have everything you wished but when you give it a chance it actually offers a much more solid and enjoyable experience overall.

FBI said:

I think it revolves around fulfilling the player's "Pokemon goals". When someone picks up a hack, they're looking for a certain experience. I think at the core, those are:
1) Building a Pokemon team from a diverse pool of Pokemon (including searching for those Pokemon)
2) Raising Pokemon to be strong -- the feeling of progress (people want longer releases!)
3) A new story, as to not feel like they're replaying a base game

With these points in mind, you can further enhance the experience with:
- QOL changes (makes the required grind repeatable)
- Features which assist with experience goals (i.e Dexnav)

As such, hacks that prevent, or hinder the above will not be successful. Such qualities include:
- Bugs which break immersion, gameplay or cause inconvenience
- Hacks which look and feel like the default game (point #3), could be from maps, music, graphics, to scripts even.
- Hacks with intrusive story lines which hinder goals #1 and #2. Culprits can be excessive dialog, mandatory brickwall-esque battles or even frustrating puzzles in succession.

Things I think people mistakenly think a good hack requires:
- Good gym puzzles (this is something the developer wants. The player is happy with solvable, non-repetitive puzzles)
- Difficulty (Players who want this impose it upon themselves with Nuzlocke and randomizer runs). Unbound did a great job of making this togglable for players who were looking for it.
- Unrelated systems (crafting [i.e DPPT's Poffins], berry growing, ect. very few players care for this. You want to tie your systems into achieving the fundamental goals without trivializing said goals i.e overpowered exp gain)

Click to expand...

This is such an articulative post on the given topic; as expected from a Rom-hacking wizard! I don't think there is scope for adding much, as he has covered every aspect of "making a Rom Hack interesting" masterfully. Nevertheless, I would like to add a smidgen of my understanding of the mentioned topic by slightly elaborating / annotating the post of FBI as I find it pretty succinct.

The core of Experience:
1. Variety in pokemon team building- This does not mean that you have to throw in all the 900+ mons in a single region hack, I discourage this practise for obvious reasons. Even if one picks 50 hand-picked pokemons from each generation, the number goes to 450 and that is enough pokemons for a single region hack in my opinion. However, if one also adds 50 something regional variants or neatly designed fakemons, then this diverse pool of pokemon will certainly maximize the variety aspect in pokemon team building.
Disclaimer: I don't endorse betamons and crappily designed fakemons.
2. Making the chosen pokemon even stronger- By giving option for nature changing, IV perfecting, egg moves, ability switching, EV perfecting and move tutors. These options will certainly keep a person glued to a hack for a while, as we all want to maximize the potential of our beloved pokemons.
3. Story- A new region is always interesting, however, if the story of this new region has elements of non-linearity, plot-twists sprinkled with surprises, then I'm really looking forward to this kind of adventure. Additionally, adding genres elements like a murder mystery, horror, slasher, zombie apocalypse etc. to the main story or the side quests might also make a hack some more interesting.

Enhancing the core experience:
1. QoL enhancements.
2. Assisting features in core experience like Dexnav, Dowsing Machine and Stat Scanner items of Unbound.
3. Mechanics enhancements like Nature Changer, Hidden Ability Switching, IV perfecting etc.
4. Graphical enhancements.

I don't want to add anything else now, as I fully agree with FBI's points regarding what not to do with a hack.
The topic of discussion was pretty interesting in itself, by the way. : )