Arcydea allows you to build class levels in multiple classes in a slightly easier manner than traditional multiclassing. Please note that these rules are extremely homebrew, and if you try to convince your DM to use them in a game outside of Arcydea they are free to point at you and laugh vigorously.
Your XP/milestone cost to reach the next level is equal to the cost to increase the level of your highest class by one.
When you achieve this next level, you can choose to advance one level in any class you choose, including in your best class.
Proficiency Bonuses: Your proficiency bonus in your chosen class is equal to the bonus provided by that class plus half the bonus of the highest-level alternate class you currently have. This applies to skills that class gives you, spell modifiers, and anything else for which 'proficiency bonus' is called for. If you have skills from multiple classes, your highest proficiency bonus is used.
To-Hit Bonus: Your highest proficiency bonus, plus half of your second-highest proficiency bonus, is used when determining your to-hit bonus.
Hit Points: You gain the hit points plus Constitution bonus for every level up you receive.
NPCs: NPCs, enemies, and villains can also be built using this system. You've been warned. Laugh at the little goblin all you like, you won't like him if he's a third level Champion, twelve level Thief.
Ability Management: Abilities that directly overlap, such as sneak attack dice from multiple sources, stack similarly to Proficiency Bonuses; the highest plus half of the second highest. Abilities that do not directly overlap, such as spell lists, inspiration dice, etc, are all available separately.
Arcane Spellcasting Level: This is considered to be the highest plus half of the second highest.
Divine Spellcasting Level: This is considered to be the highest plus half of the second highest.
Feats / ASI: These are gained at the normal levels as noted within each class.
This needs actual playtesting. It is not gestalt play, but characters developed using this system will find that they are much stronger (as are their enemies) when they have the opportunity to take levels in many different classes more readily.