I have three favourites, out of which one fits the OP bill. Kind of. So let's start with that one.

Neverwinter Nights 2 is a solid RPG and one of the add-ons (Storm of Zehir) allows you to create an entire party rather than just one character. However, I would not recommend NWN2 to anyone who doesn't know Dungeons & Dragons 3.5 Edition (its character creation/development process especially) or is unwilling to get a crash course on how it works. A pretty basic understanding is enough, but the game certainly doesn't walk you through that.

Secondly, Morrowind. Yes, the system is rigged as hell. It's rigged as two hellish hells put together, in fact. You can exploit the shit out of it, there are properly useless skills and so forth. But you can mix and match what you want to throw in, which I appreciate, and the min-maxing tickles some autistic part of my brain that I'm sure exists.

Thridly, Dark Souls. The character creation part is rather unimportant; you start off quite squishy whichever class you go for and must develop your skills (not the character's) if you are to survive, developing the character's attributes as best fits your playstyle. It's a game that you can start as one class and effectively end up a complete different one by the end. And yes, there are ways to reset your points and redistribute them if you bollocks it all up, which is a nice concession from what is otherwise a pretty unforgiving if rewarding game.

Agreed, and that's why I prefer the Divinity approach to character building, particularly D:OS 1 & 2, although Divine Divinity and Divinity 2 also allowed for a lot of experimentation. With the caveat that IIRC only D:OS 1 had you create more than one character at the start.

As for the more party-based RPGs I am familiar with, I'd probably pick Icewind Dale or The Temple of Elemental Evil, because they allow you to create a party of 5-6 different characters right from the start. The same applies to the old Goldbox games, my favorite being Forgotten Realms Unlimited Adventures, but the classes have a little less options in them compared to newer D&D games (like NWN). Not sure if any of the D&D games can keep up with Wizardy in terms of variety though, as I haven't really played that series. The Realms of Arkania games are neat as well, but I found the character creation to be somewhat of a PITA (you have to run a different exe for it, IIRC, and class choices are heavily restricted by your dice rolls - you have to know what class you're aiming for and what it requires and then roll and roll until you get the right attributes or something.

Jagged Alliance 2 XD

Yes, it is an RPG, as it has all the features for character progression (not to mention NPCs and quests). And its process of creation of your (optional) alter-ego character is a ton of fun. I think the only other time I saw something like this was Dark Heart of Uukrul

I've always liked Fallout and Fallout 2's character creation. Though you don't create a party in that one. You do have several optional additional party members to choose from, to compliment your character.

I'd also mention Neverwinter Nights, where there are scores of very different character builds you can aim for
https://nwn.fandom.com/wiki/Category:Character_builds
which you then compliment with any of the several recruitable party members in the game (only 1 extra in NWN though)

Like Loothunter Suggests, Jagged Alliance 2 is also very good, and everyone from the expansive mercenary roster to recruit from have a ton of personality:
https://www.youtube.com/watch?v=KaZkUpVv63g
They also make all kinds of comments in the game, aimed at the situation, or at each other, e.g. some mercs really like/dislike other mercs.

Hits the invert button.

You know what character creation system pretty much sucks? Eschalon.</span>

Would you like to waste 16 skill points to get a feature that most games give for free? No? Then you won't have a minimap because it requires an investment in cartography. Want good spells? Hope you're ready to sink a non-trivial investment into 3 seperate stats.
(That's not counting the magecraft/spellcraft investment.

It's a bunch of annoying rampantly redundant skills which should have been rolled into one major skill or stat instead of ruining what would have been a promising character creation system.

Also, Magic with a CK, because fuck you, that's why.

This doesn't cover the fact that a lot of builds are completely non-viable and there's no respec button.

On the flipside:
I think Avernum does a fairly competent job of bringing forth a fine stat system.
The explanation is detailed, there aren't too many to get lost in, and you a sane number of skill points to distribute each level.

D&D RPGs are probably comparable to earlier Wizardry games (1-5, but note 4's lack of character creation) in terms of variety, with later ones having more, but perhaps still not on the level of Wizardry 6-8.

D&D RPGs up through Baldur's Gate 2 allow multiclassing at character creation time, essentially offering a bunch of class choices right from the start.

I think that the two Dark Sun games (well, three, but one of them is a defunct MMO that's no longer playable) offer a bigger variety than the average D&D game, with a race that gets a bunch of natural weapons, more multi-class choices (humans can even dual-class a second time, rather than just the one time that's standard in D&D-based games that allow it), and psionics.

You mention Realms of Arkania restricting classes based off the dice roll; this also happens in the Wizardry series (excluding 4 and 8; 4 has no character creation at all, and 8 removed the dice roll, allowing you to start with any race/class combination).

That's not all that's wrong. To me, one of the worst parts about Eschalon Book 1 is that the game displays a gender field during character creation, but if you try to change it away from Male, the game gives you a BS excuse about the story "only working for a male character" or something like that. If you *really* need to force the character to be male, don't put a gender field in the character creation screen (but you really shouldn't force the player's created character to be male in the first place).

I could also complain about Lords of Xulima's character growth system, which is one of the two games (the other being Dragon Quest 8) that soured me to skill point systems. In LoX, it got to the point where I was dreading the level up screen, and that should never happen in a game with a level/XP system. (Even Cave Story's Nemesis weapon's leveling didn't feel that bad, though at least in that game you could level down your weapon by being hit.)

I'm interested in a system that allows me to create a party of 'unique' characters, with mixed classes, deep learning/skills options available, along those lines.

Appearance is not primary, but it is nice to be able to select from different portraits and even voices.

So yes, pretty much any of the dnd games such as baldurs gate, planescape, temple of elemental evil, never winter. Plus most rogue games have a fair bit of customisation. Tales of myal and ADOM on here are examples, although not party based

Mixed classes? If you don't mind having much of the "creation" being in the form of post-creation options:
* If you want a roguelike, there's Tangledeep. You can change job while keeping abilities from the previous job.
* For an SRPG, there's Fell Seal: Arbiter's Mark. (This game is a lot like Final Fantasy Tactics, if you're familiar with that.) Your characters can change class and equip the skillset of a previous class, not to mention passives and a reaction ability.

Note that character creation in these games is simple; what's not simple is how you develop the characters afterwords.

One of my favorite RPG series and also one of those that I spent way too much time creating my party lol.
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There are basically three games that come to my mind: Pathfinder: Kingmaker and Neverwinter Nights 1 and 2. Neverwinter Nights 1 is maybe not the best choice when it comes to party creation, but it shines really strong when we're talking about main character development. Neverwinter Nights 2 and Pathfinder: Kingmaker allow for deep party modifications as well, so these are probably your best choices. All three rely in their core on 3E or 3.5E D&D systems which are known for varied possibilities of character creation. Give them a shot! :)

Definitively Neverwinter Nights 2.

Dragon's Dogma: Dark Arisen doesn't have much in the way of character creation as far as stats and abilities, but the fine tuning the character model, absolutely delicious.

I'd recommend using one of the texture mods and ENB mods over at NexusMods though if you really want to enjoy the creation process.

There are some nice Male and Female specific texture mods also. It's also fun to try out a particular...ahem...mod for fun, but I can't say it here or a certain someone might flay me with alive their sharp tongue. =P

Also, it's a game that has 9 vocations that can be changed using in-game currency like Rift Crystals or Discipline Points, etc.

So, you can try out the starting vocations such as:
Fighter - your standard sword and board type, but has specific skills others don't
Mage - standard mage, but nothing standard about the spells they use (also, only class that has a healing spell)
Strider - your rogue type, can use shortbows or daggers, but oh so much more than their weapons indicate

or advanced versions (once requirements are met) such as:
Warrior - similar to Fighter, but more of a two-hander and has their own specific skills
Ranger - your typical longer range bow user, but can melee also.
Sorcerer - more destructive spells than the Mage and loses the healing spell

Then there are the hybrid vocations such as:
Mystic Knight - a cross between a Fighter and Mage, but seems to have powerful skills
Assassin - kind of like a specialized Rogue maybe?
Mystic Archer - a cross between a ranger and a mage

Each class plays differently and while I wouldn’t consider them completely unique to each other, the gameplay will feel different. Oh, and weapon types are unique to each vocation.

Sorcerer is my main class and my favorite btw, though I had to start out as a Mage for the first 10 levels.

I haven't tried the other classes, but the info is based on so much insane research I did for this game lol. Would you believe I spent an entire week customizing my own guide and regional maps just to prep for this game? lol I really hate that side of myself. :(

I'm a KB and mouse user, but this is probably one of the few games I truly enjoyed using my 360 controllers.

Oh yea, I nearly forgot to mention. You can redo your character appearance once you get a certain item or unlock another certain item. Or you can use the dinput8 mod. =P
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EDIT:
- Sorcerer info was placed in the wrong section and has been moved to the advanced vocation section.
- Replaced Sorcerer with Mystic Archer which should be the 3rd hybrid class that I forgot to add.

Thanks goes to dtgreene for pointing it out.
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Another favorite!
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I would probably use that class for the whole game, or at least most of it, because of the healing spell.