This is a random idea that came to mind after discussing a potential multiclass construction. (This thread.)

The default allowed level progressions are single-class and multiclass. Single class is just picking a class and staying with it the entire way through, while multiclass allows you to add alternate classes to your overall build.

However multiclassing is a pain to work with mechanicallly, needing a carefully chosen progression order in order for it to be feasible at any given level (though less problematic for simple multiclass dips). It's also purely a mechanical construct, and has a harder time integrating with narrative progression.

The thought I had which I have no idea how well it would work, and thus am posting the thread for commentary is a class replacement progression, designed to facilitate an intended narrative development of the character, transitioning from one class to a different class, and then back to the original (most likely in a redemption or "returning to the roots" type of arc).

Rules are:

1) You must start with at least 1 level in your intended "original" class.
2) You then gain levels in an alternate class for an arbitrary amount of time, but with some limit I would say somewhere between 5 and 10 levels, as that's the most that could reasonably be considered a "side arc". The number of levels must be determined at the time you initially diverge.
3) When you complete the predetermined levels in the alternate class, you gain your next level in your original class. At that point, all levels revert to your original class, and you lose your "alternate class" levels. (Optionally, you can keep some lesser amount of the alternate class levels, but no more than 3.)

So, from the original thread, you might progress with:

Monk 1
Monk 1/Barb 1 ⇒ Monk 1/Barb 5
Then the next level gained goes to Monk, and all Barb levels revert to Monk, leaving you at Monk 7 (or maybe Monk 6/Barb 1 or Monk 5/Barb 2). Further levels would continue to increase Monk (or other classes, if you then do standard multiclassing).

The purpose would be to represent "the period in your life when you were more a rage machine than a monk, but that you found your center again". When switching back, you abandon what you learned in the alternate class (including spells and spell slots, though I could see keeping the tome with all the wizard spells you'd copied down, if applicable; you just wouldn't be able to cast higher level spells that you had).

The intended progression would have to be coordinated with the DM. For example, a 2nd level cleric that got subverted and joined a necromantic wizard cult would need to break free of it to go back to being a full cleric, and thus needs that event to be part of the story.

It feels like it would work on the narrative side, and being somewhat just a tweak of multiclassing, doesn't seem like it would break on the mechanical side, but I'm not sure if I'm missing anything that would be a blatant abuse. It might bypass some headaches with multiclass builds, but that's almost a benefit, really, as those headaches are why I just don't like dealing with multiclassing in general.