I'm also gonna shout out A5e, but also some of the 3rd party producers for A5e.
A5e in General:
1) Modular Crunch.
You don't -have- to use the Prestige System or Destinies or Journey System or Strongholds in order to play the game. However each of them is it's own well designed system structure that you can pick up and apply at any point to your game world and it can blend in very smoothly.
Combat Maneuvers are definitely crunchier... but you can run a 5e class in an A5e game and it'll play pretty similar with regards to balance outside of the 3 pillars.
2) 3 Pillar Play.
WotC has kind of nodded toward 3 pillar play by making some classes good at a specific pillar or another. Rogues and Spellcasters and Rangers are good at Exploration, Bards and Warlocks and Spellcasters are good at Socialization, and everyone but particularly Spellcasters are good at Combat.
A5e was designed from the ground up so everyone would have 3 pillar play options from the get go. Your Fighter gets exploration centric Knacks and socialization centric class features with multiple options for how to approach their social benefit.
3) Character Option Exponential Increase.
5e has started generally to drift away from race toward something A5e like (they'll probably get there in 7th or 8th edition) but A5e looked at Pinnacle Entertainment Group's 3pp supplement "Ancestry and Culture" for 5e and went "That's so brilliant it needs to be a core rule item rather than a 3rd party product and WotC is too foolish to realize it!" and went for it.
That book won two Ennies for being the Best Supplement and Best Electronic Product. It -nearly- won Best Product of 2021. It lost to Alice is Missing because that game is amazing and creepy and Brancalonia because we need more Spaghetti Fantasy.
You pick your Heritage (let's say Elf) and decide you wanna make yourself a half-elf (let's say Dwarf). So you take the Elf Heritage and a Dwarf Gift or the Dwarf Heritage and an Elf Gift. Then you pick your culture (Wanna be a half-drow half dwarf? Go for the Dark Elf Culture. Wanna be a surface Dwelf? Go for the Hill Dwarf culture. Don't wanna have your Dwelf defined by their ancestry and just live in a big town? Cosmopolitan culture or Circus or Wartorn or...) THEN you pick your Background. Then Destiny. Every time anyone adds a new Heritage or Culture or Background it's an exponential increase of character options that are all pretty fairly balanced!
And then for the 3rd Party Publishers around A5e.
1) Purple Martin Games
Peter Martin is kind of the Godfather of the A5e 3rd Party Publishers. Not in the criminal sense, just in the sense that he's constantly looking out for all of us and for new talent to bring into the 3pp space to foster and grow the best possible game design.
He's also, by far, the most prolific of all the 3pp. His drive for excellence and system mastery makes him amazing at exploring what the game can do and helping others to see it. His Manual of Adventurous Resources: Complete is one of the biggest products in the 3rd party space and includes work from some brilliant contributors.
He's also quite insane in his endless endeavors to bind the community together which we all absolutely adore about him. He's created Synergy Feats (multiclass-supporting feats) for every 3rd party class option from the Elementalist to the Witch. Including all the 3rd party classes he doesn't have in his books... like the Esper, Scholar, and Witch.
He also got a bunch of us into 3rd party production. And took up my idea of Motif Classes to build out into it's own full product, which is fantastic.
Did I mention that he does profit-share with everyone whose work he uses? Rather than pay us a flat 5 cents a word or something similar, every single sale of a product has automated fractional payout to everyone involved in the product for shares in the profits. He's almost standardized that structure for the 3rd party publisher community which is -very- cool and allows us all to make sure that any content needed within a given product can be there.
Someone wants to do a Paranormal Power Supplement and include the entire Esper Class and all Powers within it? Sounds great! And good odds they'll pay me for my work when they reprint it so someone who buys their product doesn't -also- have to buy Paranormal Power to use it, but I still get paid for my efforts. It's great!
2) DM Sarah
With years of experience as a professional DM, Sarah Breyfogle has worked her way through the 5e and A5e systems, worked to upgrade her professional tables from 5e to A5e, and helped to write the freaking rulebooks in the first place! Her book "Stranger Sights: Challenges for 5e and Advanced 5e" makes excellent use of her masters degree in communication and culture to provide amazing new exploration challenges to the game. I highly recommend it and also her streams wherever you can find them!
Also, Lady Lyra's Blade? Eeeeeeeee! Adventure for 5th to 7th for a great easy set of sessions. The whole adventure is super modular for different settings and easy to pick up as a quick aside in a longer campaign without feeling entirely "Derailing".
3) PJ Coffey
PJ's work on systems design and delving into the math behind the game is fantastic. Their books mostly focus on expanding DM options for creating new and balanced content against the game, exploring the functions and levers thoroughly, and developing new and simple systems to handle various situations that 5e and A5e might not handle 'Well' from a storytelling standpoint even when it works fine from a mechanical standpoint.
Like escaping encounters. 5e doesn't offer much other than "Compare your movement speed to your enemy's movement speed and see if you can get away. Add in some ability checks, too, maybe. But anyone with under 30ft movement speed is almost always gonna get caught!" which is fairly realistic, but narratively unsatisfying at best.
4) Plant Witch Press
While, personally, I'm not big into the cottagecore aesthetic and style (I'm more Halloween/Spooky), Jessy Mullins' work in Hearth and Home: Cozy Cottage Origins is stellar. With enough heritages to do an entire campaign set in a cozy forest with birds and foxes and halflings and gnomes and crystal and plant creatures hanging out with flying sprites... it's sincerely super cool. I've got a copy burning in my DTRPG "Wallet" of potential games and while I don't generally do cottagecore I could absolutely use this book and all the material in it to do some kind of "Unseelie Takeover" storyline from the perspective of a bunch of adorable animal folks living in a Fey Forest of some sort. It's been percolating for a while, lemme tell you.
But she's also got Paradigms of Skill which is a fantastic product for people who like critical expansions and even took the time to create a 3rd party product for Lars Torgeson's Witch!
5) Roll Them Bones Gaming
Speaking of Lars Torgeson! Heroes Old and New introduces a Scholar class which allows for a "Less Magical" Intelligence based character who is very flexible in what they do, and Mysterious and Marvelous Miscellanea introduces the Witch for a great morality play structure between good, evil, and balance.
Also, c'mon, if I'm going to do a cottagecore nightmare campaign with the Unseelie you -know- that I've gotta have Witches as a playable option!
There's more great 3rd party publishers for A5e beyond these 5 (I like to include myself among them, for example), but here's the tea on the first three:
They helped to write the Core Rules for A5e. So when you're getting 3rd party content not published by EN Publishing, itself, you're still getting material written by the same designers. (And we're all here on the forums, too, answering questions and promoting new material or ideas!)
And practically all of us have, or routinely do, get published in the Gate Pass Gazette, a monthly periodical that EN Publishing puts out of new content submitted by freelancers that gets editorial approval before EN puts it out as official, new, A5e content. $10 a month gets you 4 articles at 2,000 words each with new items, heritages, cultures, backgrounds, systems, archetypes, encounters, and more.
WotC could -never- support a 3rd Party Community so well as EN Publishing does. Nor care about it's community like EN Publishing does. I will always feel grateful to
@Morrus for building this wonderful community for me to be a part of.