Reading Overlord really makes me wonder if a video game like Yggdrasil would be possible. Aside from the obvious fact that it's a futuristic VR game, I feel like some of the character customization features would be interesting to see attempted (outside of a tabletop game). It would be cumbersome to go through everything, so I'll stick to the leveling system for the sake of this post.
So the standard design in MMOs these days is to have the player choose one of a handful of classes at the start, then grind out enough experience points to level up, and pick out the abilities they want from that specialization.
In a Yggdrasil-like system, the role of classes would be completely reversed. They would not be a handful of tracks chosen at the beginning of a game, but rather a set of specialties that could be gained organically as the game is played. The best way I can think of to explain is with examples.
Say that you choose to pursue the "fighter" class, and find one of the in-game trainers for it. "Fighter" would probably be one of the basic specialties, so there would be no mandatory requirements to begin leveling it. Instead of simply grinding for experience points, to level up as a "fighter" you would have to complete specific tasks, use fighting-related skills, and complete quests for your "fighter" class trainer. When you do level up, you gain the predetermined stat increases and abilities of a "fighter" of your level. This may seem like it limits the player's freedom to customize their character, however, players can still do so by choosing what classes they choose to level.
For example, after gaining 5 levels as a "fighter," maybe a player might choose to meet up with a "magician" trainer, then gain 5 levels in that class, making them level 10 overall. This would come with upsides and downsides. On one hand, they'd probably be at a serious disadvantage against someone who poured 10 levels into one class, since their stats would be more spread out. On the other hand, they'd be able to use the special abilities of a level 5 fighter and level 5 magician, which could be made effective in the hands of a skilled player.
And then there would be advanced specialties to consider. Let's say there's a class called "spellsword" that has a starting requirement of 5 levels in the "fighter" class and 5 levels in the "magician" class. In which case the aforementioned player would have the option of getting quests from a "spellsword" trainer, and gain access to the unique abilities of the "spellsword" class.
On the other hand, someone who reached level 10 as a "fighter" might have their pick of warrior specialties to move onto. Maybe they'd look for a "knight" trainer for a defense-oriented build, or a "marauder" trainer for an offense-oriented build, or a "monk" trainer for a H2H build, or maybe they'd just keep on leveling the basic "fighter" class for a more well-rounded build.
Ideally, there would be multiple layers of advanced classes. For example, let's say you wanted to be a "beastmaster," a powerful class that gives you the ability to keep a set number of magical beasts with special abilities as your companions. Starting the "beastmaster" class requires at least 5 levels in the "ranger" class, and at least 5 levels in the "commander" class.
-->To qualify for training as a "ranger," you would need 10 levels in the "rogue" class, and 5 levels in either the "knight" or "marauder" classes, both of which we've already established require 10 levels in the "fighter" class to start.
--> To qualify for training as a "commander," you would need 5 levels in the "bard" class, which itself requires 5 levels in each of the three basic specialties--"fighter," "magician," and "rogue"--to qualify for.
So in other words, to become a "beastmaster," a player would have to be level 50 at the absolute minimum, and to become a masterwork "beastmaster" they'd need to be level 65 (if we operate under Yggdrasil's assumption that each class has 15 available levels). This is all assuming they only took the classes that are necessary to become one, of course; playing more practically, someone aiming to be a "beastmaster" might not become one until the end-game. Additionally, it stands to reason that the more advanced the class is, the longer and more elaborate the quests to start and level up those classes will be. (Since, even if you had the necessary skills, you obviously wouldn't be trained as, say, a Paladin in a fantasy setting without becoming a part of a Paladin Order's politics. In that way, picking a class would be like joining a guild, and the more advanced the class, the more exclusive and important the guild you're joining is...? Well, that's the idea, anyways).
Now, if someone wants one of these more special classes, some planning will have to be involved... but that's part of the point. Also, players can be just as (if not more) effective by continuing to level the basic and intermediate classes, if they don't want to bother with more complex ones (although they might miss out on some of the game's most fun abilities to use).
Ultimately, while this kind of system would be immensely difficult to pull off well, I don't think it's such a bad idea. Allowing the player to mix and match multiple classes out of dozens (in ways that may or may not synergize well) would make roles in multiplayer games far more interesting, in both PVE and PVP. Additionally, it would make for great replay value, since not only would the play style change for different builds, but the questlines required for those builds would also be different.