[HR] Multiclassing system

This started out as a concept for a sci-fi class system, but I don’t feel like it worked very well for that. (It doesn’t work for me as a core class system, only as an expansion to an existing working system.) However, I do think it’s a system that could work for a more lifepath-type multiclassing than the core ACKS multiclass method of ‘build a new class for it, then have picked that class at first level’.

At first level, you select a class as normal. This class is your primary class. If you so choose, you may simply advance in your primary class without any change from the core rules; this system is compatible with single-classed characters without any change.

A character may train themselves to enter a new class. Doing so requires 1,000 XP either performing activities of their new class or being trained by a member of their new class. (The exact requirements are left to the Judge’s discretion. Most importantly, 1,000 XP vanishes.) Once this is achieved, the character gains their first level in their first secondary class. At this point, the character will have two classes, both of which are active classes. A character may have at most three active classes, one of which will be their primary class and two of which will be secondary classes. When a character gains XP, they may choose to apply it to any single one of their active classes.

A character may train to access more than three classes. Their fourth and subsequent classes will initially be inactive. A character may spend a month training and exercising, along with the expenditure of 1,000 XP, to alter their active and inactive classes. After this month, they may select any set of up to three classes that they have access to (one primary class and two secondary classes) as their active classes.

To actually determine the character’s abilities, the rule of thumb is best of. Abilities (including hit points) from different classes do not stack. For all level-dependent variables (such as GP threshold), the character’s level is considered to be their highest level class. (As a general rule, for reasons that will be mentioned later, this will be the character’s primary class.) However, if the character has two lower-level active classes that are both within two levels of the highest-level class, add one to the character’s effective level for such purposes.

The maximum level of each active class is reduced by two for each active class beyond the first. (That is, two active classes; max level 12. Three, max level 10. Assuming, of course, that they all started with a max level of 14.)

The character’s hit points should be rolled separately for each class, according to the individual level of each class, and their total hit points are equal to whichever single class has the most hit points. (A fighter4/mage6, for example, would roll 4d8 for the fighter and 6d4 for the mage. If, hypothetically, those rolls came out to be 24 for the fighter and 20 for the mage, the character would have 24 hit points.)

The character’s attack throw is the best attack throw value from any of their classes.

The character may cast arcane and divine spells (as well as any other types of spells) as the best of any of their classes. (For example, an elven nightblade8/elven enchanter 3 would have the spellcasting abilities of the nightblade8, as that is better than the enchanter3. Some sort of horrific Nobiran Wonderworker5/cleric6/mage7, a combination that I have no clue why anyone would waste XP on, would cast spells as a cleric6/mage7. Note: see optional rules.)

The character has the weapon and armor proficiencies of their primary class.

The character has the saving throws of their primary class.

The character gains general proficiencies based off their highest class level.

The character gains class proficiencies only from their primary class. (A character who changes their primary class loses class proficiencies from their former primary class and gains them for their new one. It is up to the Judge’s discretion whether or not this allows a full reselection, or if a character who goes back and forth should be ‘locked in’ to their former selections. Personally, I would run the latter.)

A character may not gain any benefit from classes that are excessively lower level than their overall power. A character whose effective level is 5th or higher gains no benefit from a first level class. A character of 9th level or higher gains no benefit from classes of levels 1-3. A character who has reached maximum level in an active class gains no benefit from classes of levels 1-5.

Optional rules:
-A character must apply XP only to their primary class.
-A character may have a varied number of active classes. (Limiting characters to a single active class is similar to 2E dual-classing, I note.) I recommend making sure there’s a limit to constrain one-level dips, as opposed to allowing any number of active classes.
-A character may only gain spells from a single active spellcasting class, regardless of type. (This would mean that, unless they are a wonderworker or similar class, no one can cast both arcane and divine spells.) (I have gone back and forth on whether or not this rule should be ‘core’. It is not necessary to the system functioning, so it’s optional, but personally I don’t think I’d run the system without this rule in place.)
-Advancement in a secondary class costs additional XP (beyond the 1,000 to train it). Suggestions include 1,000 * level, double, or triple.
-Characters are required to split XP evenly between all active classes, instead of assigning it to the class of their choice. (To create 2E multiclassing, combine this with a rule that no character may learn a new class after level 1. That is, their first X,000 XP must be spent on learning new classes, and only then can they rise to level 2.)
-At the Judge’s discretion, special abilities gained at first level may function only when in a primary class, or may require additional class levels to unlock.
-Optional saving throw calculations: Begin with the level 1 saving throws of your primary class, then improve all numbers by 1 for each point by which the character’s attack throw is better than 10+.
-Optional proficiency calculations: Gain one class proficiency of your primary class at first level, then an additional class proficiency of your primary class for each 2 points by which the character’s attack throw is better than 10+. (Note that this has a rounding error, because the actual class method is weird and gives you the proficiency before the attack throw actually increases! Fighters, for example, have a 9+ attack throw at 3rd level, but gain their next proficiency then, because at 3rd level they are done with 9+ and their next level will be 8+. I consider this rounding error a single-classing benefit.)

PS - I am aware that you can take the guts of this system and remove classes entirely, buying your class values with XP. That variant is not yet in a state ready to be posted, though it is roughly concepted out in my head.