You are close to the idea but very far. Each class is leveled up separate depending on how you want to make your character as he levels up, like how a lv12 human can be lv8 druid, and a lv4 monk. What I'm getting at is the same concept, but you only get the basic features once.
Ok so you said 3 archtypes well that means there are four tables. The main and three archtypes. you can only get the abilities from each of them once so the main table can only be done once, so say you have a lv7 druid circle of the land who is also lv 5 in circle of the moon. he only gets up to level 7 on the main table but get his 12d8+con hp and get the befits of his circles at the same time.
As for stopping at level 20, you're right when it comes down to just the first arche type but if you wanted to master it you would need to be level 17 to get the first archtype done, then level 34 for the second, and 51 for the third, and another 3 for the main table this bring you to level 54, and now you can say that you mastered the class.
And just to make this clear. if you think this is nuts. it is. your dm will have to constantly modify the baddies and goals even be unfair, but as it says in the monster manual those are "base stats"
I don't think it's nuts per se, but it does involve a lot of dead levels where you don't gain any real benefits for leveling (beyond hp). Assuming you allow another character to multiclass normally (ie, at level 54 he might be a Paladin 20 / Sorcerer 20 / Warlock 14) the normal multiclass character will be
much more powerful than the guys specializing in their existing class. Similarly, a Druid Land 7 / Moon 5 is not going to compare favorably against a Paladin 12, because while he has the hp of a 12th level character his features are only a little better than those of a 7th level druid.
In my opinion, making the player take all those dead levels makes the cost too great. On the other hand, just letting them take the archetype levels is probably too strong (ie, a 3rd level Druid who is Land 2 / Moon 2). Under the latter, an assassin rogue could dip one level of arcane trickster to gain all of the trickster's spellcasting progression for the relatively low cost of one level (Shield alone makes this worth it).
[MENTION=20323]Quickleaf[/MENTION] has a good idea, although I wouldn't allow it to work quite like he describes. The reason is that many classes have utility levels built into the archetypes. For example, allowing a 6th level Totem Warrior Barbarian to take Frenzy instead of Aspect of the Beast is arguably overpowered.
The simplest solution would be to allow characters to gain archetype abilities via epic boons. So a rouge assassin 20 could gain the arcane trickster's spellcasting and legerdemain features at "21st" level, magical ambush at "22nd", versatile trickster at "23rd", and spell thief at "24th". He can master all three archetypes by "28th" level. While archetype abilities are arguably better than many epic boons, since everyone has access to these options it should largely balance out. Epic D&D is fairly wahoo anyway, with balance being less of a significant factor than it is at low levels.
While it is isn't exactly what you're looking, I created the following feat for my own game. I haven't had any issues with it, although I've only had it used twice (once by a druid [land/moon] and once by a wizard [conjuration/evocation]). It is, however, rather complex and requires some DM adjudication.
Prodigy
You are a master of two paths and your ability to think outside the box and innate affinity to your class has allowed you to learn to blend training from more than one subclass of your class. You gain the following benefits:
You gain proficiency in one additional skill or tool of your choice from your class list.
Choose a second subclass from a class you already have levels in and that has more than one subclass. Doing so grants you the following benefits:
When you gain a new feature from your subclass after taking this feat, you may choose which of your subclasses to gain features from. (You do not gain the benefits of both.) You are considered to be of that subclass until the next level at which your subclass grants a feature (this pertains to abilities the subclass grants in between feature levels).
For example, a Druid could become both a Land and a Moon druid. At 2nd level, you could choose to gain either the Circle of the Land features (Bonus Cantrip and Natural Recovery) or the Circle of the Moon Features (Combat Wild Shape and the 2nd level benefits of Circle Forms). If you choose Land druid, you also gain Circle Spells at levels 3rd and 5th. At 6th level, you can again choose between Land (Land’s Stride) and Moon (Primal Strike and the 6th level benefits of Circle Forms). If you choose Land at 6th level, you will also gain the 7th and 9th level Circle Spells. At 10th level you can choose between Nature’s Ward and Elemental Wild Shape, and at 14th level you may choose between Nature’s Sanctuary and Thousand Forms.
Let’s assume you are playing a Druid with this feat and select Circle of the Moon at 2nd level. You gain Combat Wild Shape and Circle Forms. Then, at 6th level, you choose Circle of the Land. You gain Land’s Stride and will gain the bonus spells at 7th and 9th level appropriate to the land type you chose. However, you are still limited to CR 1 beast shapes, since you have not reached 6th level as far as your Circle of the Moon features are concerned. At 10th level you take Circle of the Moon again and gain Elemental Wild Shape. Additionally, you can now assume the form of CR 2 beasts since you are effectively 6th level in the Circle of the Moon. At 13th level you gain the ability to become a CR 3 beast. At 14th level, if you choose Circle of the Land you gain Nature’s Sanctuary and your Wild Shape does not increase beyond CR 3. If you choose Circle of the Moon you gain Thousand Forms, and your Wild Shape CR increases to 4 at 16th level and 5 at 19th level.
As another example, a Fighter could choose to be both a Champion and Eldritch Knight. At 3rd level assume you choose Champion, gaining Improved Critical. At 7th level, you choose Eldritch Knight, gaining War Magic and Spellcasting. However, your spellcasting starts as per a 3rd level Eldritch Knight, because you were not an Eldritch Knight from levels 3-6, and therefore did not gain the benefits thereof. At 9th level, you will have the equivalent casting of a 5th level Eldritch Knight. Continuing on, at 10th level you take Champion, gaining Additional Fighting Style. Once again, because you are not an Eldritch Knight during this period, your Spellcasting will not improve from levels 10 to 14. At 15th level, you once again switch to Eldritch Knight, gaining Arcane Charge and improving your Spellcasting to the equivalent of a 6th level Eldritch Knight. At level 17, your Spellcasting will have improved to the equivalent of a level 8th level Eldritch Knight (allowing you to choose one spell from any school of magic, per your Spellcasting feature). If you choose Eldritch Knight at level 18, you gain Improved War Magic, and by 20th level you will be the equivalent of 11th level Eldritch Knight in Spellcasting ability. If you instead choose Champion at 18th level, you gain Survivor and your Spellcasting capabilities will remain the equivalent of an 8th level Eldritch Knight.
