Mewgenics has a very unique form of multiclassing. Cats are largely willful creatures and you only have so much control over how they level up, drafting their abilities and passives. But if you breed your cats in purposefully decorated and populated environments, they can sometimes inherit off-class abilities and passives from their parents, and then you can slap on a class collar you think might mesh well with those abilities. Its always a gamble, but sometimes you can come out with Scott Steiner esque genetic freaks with 141% chance of winning.

Final Fantasy Tactics does classes (jobs) wonderfully imho. Also let's you mix and match abilities for multi classing.

Dragon's Dogma doesn't have a ton of options but it's also a fun system imho, and has some hybrid classes.

Not my fav, but I have a soft spot for Torchlight 2 for sentimental reasons.

Final Fantasy 14 has to be up there for me as well, even though I didn't play it much. The not playing it much had more to with my lifestyle not meshing well with an MMO anymore, not with the great job system it had.

Guild wars 1. Its a skillbar mmorpg, 1 time purchase for $10. Its back in active development by a small team.

You get a primary profession like warrior and a secondary one like ranger, monk that can be swapped at any outpost. This allows you to get access to a much wider array of skils to build for the situation.

Each profession has a primary stat so you dont have warrior/monks out healing monks because they dont have the devine favour stat.

But there is still a ton of build diversity and role diversity.

I'm playing a Paragon/warrior so I run a lot of DPS support shouts from the paragon class and weapon focused skills from the warrior class as well.

Gotta play Grim Dawn

If you don't need super fancy graphics and don't mind some walls of text, both of Owlcat's Pathfinder games, Kingmaker and Wrath of the Righteous, are absurdly intricate character builders. There are probably 20 classes, close to 100 subclasses? You can generally mix and match them as you wish other than not being able to take two sublasses of the same class.

Both games also include a roguelike mode DLC that can be played separate from the main campaign, which are mostly combat and great for testing new builds.

Do yourself a favor and install the mods that let you define your standard buff set and cast it as a macro. Otherwise you're gonna spend a lot of time manual buffing between fights.

KM buffbot: https://www.nexusmods.com/pathfinderkingmaker/mods/119

WOTR bubblebuffs: https://www.nexusmods.com/pathfinderwrathoftherighteous/mods/195

I really enjoyed the previous big RPGs by larian, Divinity Original sin 1/2.

Apart from the gameplay just being fantastic, it has a very open class system where you put points into different abilities and you can freely mix and match whatever you want.

It also puts a huge emphasis on environmental hazards and how they interact, which makes the combat feel very novel.

Look into the Disgaea series (JRPG) and Grim Dawn if you want to play the spiritual successor to Titan Quest.

I would love if someone made a game that is the midpoint in complexity between PoE and Grim Dawn, but I feel like I might be dreaming for awhile

one thing i'll add is that i've heard good things about guild wars 1, rift, and ff11 but have struggled to break into them. the only MMO that's ever really stuck with me is wow, and i play that super casually now. i had fun on ascension (customizable class private server) for a week or two but the classic experience doesn't grip me like it used to.

For this one specific thing? The Elder Scrolls and Ultima series. As well as Shadowrun in the TTRPG space.

They don't have classes. They just have attributes and skills, so you can make whatever class you want just mix and matching stats and skills together.

champions online and star trek online have massive build options. Both costuming and systems with champions using the champions system and having a freeform option that is only limited by having to have some lower abilities to unlock higher ones and then sto has the initial jobs classes of tactical, science, enginering but then any class can fly any ship of the same class options but then they added specializations that you can level and mix and match one primary and one secondary and the ships also got specialization and then universal modules and gear. its crazy infinite combos. neverwinter kinda had a lot to (other cryptic mmo) but compared to champions and sto options its hardly anything. I have to say I so wish the company that made baldurs gate would make something based on pf2.

I liked Etrian Odyssey IV's subclass flexibility so much that I had a hard time going back to Etrian Odyssey Untold without it.

Not an expert on RPG but I liked Bravely Default a lot in that regard because rather than multi class you kinda collected classes and switching them on the go made the gameplay so much more varied.

Istaria may be old and a bit obscure but it is a personal favorite of mine. It's a small mmorpg with many playable races (and playable dragons!) that features a multiclassing system. A single humanoid character can basically level any class(es) and mix and match the abilities for a custom build.

You already named Baldur's Gate 3, and I don't think I've found one more interesting or elegant than D&D 5e. Maybe other folks are more well-traveled than I am, but I've played most of your examples, and 5e still takes the cake. You can still do better, especially since not every attribute is equally important across classes, but it still makes for fun synergies as is.

I loved Wildlands; not necessarily the multiclassing, but the fact it was just Borderlands in a fantasy setting, and they made it work. Really well, imo.

A game I like that has multiclassing is Kingdoms of Amalur. It's pretty arcade-y and kinda simple, but it's also just really fun to play.