This modification to the DnD 5e system is definitely not RAW at all, and your local DM is in their rights to look at you and scoff if you attempt to get them to accept it. It is fitting to the history and spirit of Arcydea, however, and as such we offer it as an option.
This is meant mostly to make it so that a character who wants to broaden their focus can do so, once they have established themselves in their given field. Multi-classing is not available until character level 5, and the following alternate rules are used:
Levels earned on additional classes beyond the first counts for only half a character level per level, rounded down. This means that a 5th level fighter, 2nd level rogue, 1st level wizard would be considered to have a CL of 6.
Adding a new class: This requires 6,500 experience for the first level in that class and each level thereafter up to 5th, and then the cost of the next character level thereafter (counting the current character level rounded down, as noted above). For example, a 5th level fighter/3rd level rogue would be considered CL 6, and need to raise 9,000 experience points to increase his fighter level to 6 or his rogue level to 4. Either way, his CL would become 7, and his next level in either class would require 11,000 experience points. You still must fulfill any prerequisites required, and you gain only the Multiclassing Proficiencies on PHB p164; it's just cheaper to attain and doesn't impact your character level quite as much.
Adding an archetype to an existing class: In order to benefit from multiple archetypes, you have to take the archetype as though it were a separate class as above – which means you will need between 13,000 and 19,500 experience points expended to take advantage of it.
What about my hit points?!: Take heart! You do gain hit points for each class level! The only significant change is that your character level increases at half the normal rate when you take a level in an alternate class.
This is a horrible idea. My pre-existing character was a level 5 fighter, level 5 rogue, and now he counts as CL 8 instead of 10!: That is why this is undergoing playtesting. For now, starting characters are treated under the same RAW rules as normal; it's when they work on increasing their level that this comes into play.