I have a player that wants to build a Dragonborn Outlander Ranger/Sorcerer, with Slaad as the first Favored Enemy.
The archetypes for the Sorcerer are Draconic Bloodline and Wild Magic. Being Dragonborn fits the Draconic Bloodline. And her backstory, in which she was used in a Slaadi ritual, fits rather well with the Wild Magic archetype. As such, the player would like both archetypes.
My thoughts are that I will simply let them Multiclass in Sorcerer twice, thus becoming Rang 3/Sorc (Dragon) 1/Sorc (Wild) 1. While I realize this goes against the standard rules, I can't see where it conflicts. So, my question is this:
Are there any other guidelines for acquiring multiple archetypes within the D&D 5th edition class system that I am missing?
Best Answer
Using the books' rules, no, you can't.
You can only choose your archetype once, when you achieve that level (e.g. 3rd lvl Fighter). The only way to get another archetype would be leveling the same class from 1st level again, doing something similar to a multiclass Fighter/Fighter.
Although, as mentioned here, multiclassing rules in PHB specifically state
And in Sage Advice
(Just explaining to people that might get here to question the same thing and don't know why it is against standard rules, as mentioned in the text)
Hopefully, the books aren't everything
I know the same about specific official guidelines to multiple archetypes as you - I don't know any. And yet, I've created some NPCs and let my players create some PCs with multiple archetypes, although admittedly some turned out being broken. Anyway I might help:
Personally, I would make them take the Draconic Bloodline archetype and just give the Wild Surge feature for free. Balance-wise it is does not seem broken to me, although I do have less experience with Wild Magic Sorcerers than almost every other subclass.