Oh, that's a hard one for me. Usually, most hacks generally don't appeal to me. I usually look for hacks that not only possess a large degree of polish and integrity, but something that's original and delves away from the main norm of Pokémon. After a while, all hacks begins to feel same-y to me. It's a large reason why I took to Mirage of Tales so much. It felt fresh and original.
I think the whole thing about hacks feeling "official" stems from the hack not shoving fanservice down our throats. I mean, you can tell a hack is a hack when you all-of-a-sudden see Ash and Misty chill in a game where the plot was never revolved around them to begin with. Where they felt forced in as a nod rather than actually needed. You can tell a hack's a hack when you see glaring issues like boring/poorly-made maps, boring/absolutely stupid plotlines, horrible usage of gameplay features. It's stuff that otherwise wouldn't pass @ Game Freak.
There's that time, love, and consideration that should be put in every hack for it to be anywhere considered as "Official' as the canon titles. I think what makes any Pokémon game official is immersion. In my opinion, immersion is what Pokémon's all about. Your protagonist is silent so that you can project your own image on the character. You feel like you're going on the journey, not that your playable character is going on a journey. You have the choice between what team you craft and what you do with that team. All of that, and there's a degree of polish and love that makes sure that this immersion isn't broken.
When you play hacks and become fully invested, has there been that time when you ran into a big bug and think "Oh yeah, this is ROM Hack". Did you ever think this way when your player character spoke? How about when there's a gigantic steep in diificulty brought upon by poor design choices?
I think when the hack starts to show its flaws and present itself as if the creator didn't care and didn't keep steady attention to what's immersive and what would break the immersion, you ultimately lose that feeling of "Official" because you know that had this been Game Freak's game, there would've been more attention put into the small things that tie the world together, not just a bunch of insane ideas execueted poorly.
The manner in which Pokémon Mystery Dungeon deviated from the main series can be learned from – and I'm not trying to limit that to the whole "world of Pokémon without humans" trope. It's the notion that they made something different and
good that connected astoundingly with its fanbase... more so than any specific plotline you could rip from, you know?
I think what makes a good, "official quality" Pokémon game really sits on having all of your bases covered. That means story, graphics, mechanics, and Pokémon itself. While I know that's certainly asking a lot of most ROM hack developers, some hacks don't even have
some of their bases covered, you know? A really
good hack that's thoroughly enjoyable and fresh to play is something that brings new tweaks and new pieces of content to the table while retaining the core of the game that people know and love. The core of the game is the adventure, it's the battle mechanics – it's
not necessarily "becoming the w0rlds græt3st p0k3man m4st4r and battling the evil team with no realistic motives, getting all da badges and becoming the ch4mp while having ALL 840 POKEMON". Game Freak has always had us covered with that canon.
Another thread on here was discussing the ramifications of modding games, and earlier today I was watching a video from
ZeroEmpires promoting a "balance patch" for Age of Empires II HD, and it got me thinking... maybe these sorts of things are what we're looking for. Granted its for RTSes and not RPGs, it's still an example. Ignoring popularity and media placement that hacks like Glazed take advantage of, the game itself presents a good experience not just for having X number of predefined features that people like playing with, but also because it hits relatively close to that
mark of a variant original that is most attractive in any fan-based game, really. I've covered in the past that we have a lot of unrealized potential with Pokémon ROM hacks, and I think this holds true here as well — while it's still well on the board, Glazed didn't hit the bullseye (so to speak) of that target of a good fan game in my opinion; other hacks can still get plenty closer. But alas, I digress.
In a nutshell, a good "official" or "official-worthy" game is something that modifies the peripheral aspects of the game while retaining most of the core behind the originals, and the potential for hacks that achieve this is only somewhat realized as of now. It's debatable how close you can get to the center of the core aspects, as all of the main series games only remove so much, but PMD successfully stripped more, so I dunno. I think there is a limit of how much we can change for sure.