I do like the idea of dual and multiclassing similar to AD&D 2nd and 3rd edition. However, given the limitations of my own abilities, I ended up with a class and sub-class system in the main game I am doing (but the game itself gets really complicated to do with my limited skills, so I might put it aside and do a battle-focused short game to show off the battle mechanics first and as extra practice).
In my world, people have certain base classes (I have designed about 18 so far), and can progress through these classes up to the 3rd tier, with the 2nd tier being the key diverging factor (my base Mage class is the most divergent at 5x 2nd tier classes, and has a total of about 100 skills, while my base Mentalist (psionic powers) class has the next most skills, although it is about as divergent as my tank-based classes at 3x 2nd tier class). I am also using Yanfly's Row Formation plugin, and each class performs best in different rows. So, I will list down the my base classes based on rows.
1st and 2nd rows are reserved for summon actors (I initially used SRD Summon Core, but due to some plugin conflict, I ended up eventing the summons instead), with the 1st row being a lot of tankier than the 2nd row
3rd rows are the tank classes, each with their own style of tanking (I also used Ramza's Shield Block and Parry Chance plugins), so they can either simply have high defense and HP, able to use shields, high evasion, or can parry
4th row is where I get the main nukers like my Mage and Mentalist (although depending on class progression, they can become DOT specialist, which are good against high defense enemies, buffers/debuffers, etc), buffer/monster interaction classes like the Hunter (currently, the main monster interaction is that without this class, the party cannot recruit animals or monsters into the party), weak but sustained buffs/DOT/debuffs/heals from the Performer (who can also go DPS route as a Dancer, or keep up their niche with Bard progression), and a specialist healer in the Medic (but as they uses science, nothing too fancy) who can also progress in the Chemist route and become a monster with their status attacks
5th row is the range DPS mostly i.e. the Sniper and Tinkerer classes (using Bows+Crossbows and Firearms respectively, with Tinkerer also having some "mage" type abilities through technology and able to progress similarly), and nukers like the Dilettante (which are capable of all sorts of skills, but they have lower stats, crap defense, and are also Crossbow specialist with their own physical skills), Scholar (basically the Blue Mages of my world); Melder (similar to the Dilettante, but focus only on magic and psionic skills and no physical or science/technological skills, making them less versatile, but also easier to build and have better stats)
6th row is almost pure support, namely the Healer who specialize in "mage" type healing (so are a lot more powerful than Medic, but crappy offense and only have 2 attack skills, and both same element, making their offense non-existent if they meet enemies that null or absorb that element), Acolyte who can pray for various effects, but need to build up their power in combat, so the more powerful skills can be really infeasible (priest in training, and since my world is polytheistic, there are many deities so many effects can happen depending on the deity you pray to), and the Tactician, who is mostly there to give some buffs, some heals, but have really unique buffs that applies party wide, but also 1 unique DPS skill that does damage on an element that is very rare to have resistant to, being the physical damage equivalent of an Infinity+1 element
Add to that, there are various subclass available based on the base classes, which can supplement the skills (again, my Mage base class has the most variety, in part because of the 100 skills).
However, no 2 classes is the entirely the same, so, even if they seem similar in the base classes, the subclasses can change them mechanically or add abilities that cannot be found. One example is that my Mentalist class can be taken up the Kinesticist route, which is elemental base, and have skills that appear similar to Mage classes if taken up the Sorcerer or Shaman route (elemental nuker and elemental DOT respectively), but they are unique in that their skills can be toggle between single or multi-target (Sorcerers have to learn two different skills for that function, because Magic A is Magic A), and they have different elemental DOT compared to the Shaman. Therefore, even if roles are duplicated across characters, it is not disadvantageous at all, because in the case of multiple elemental DOT, they stack in my game, and a Kineticist and a Shaman can basically load up bosses with enough DOT to kill them in a few turns if they so choose to.