TwoSix
Dirty, realism-hating munchkin powergamer
Player buy-in is definitely my chief concern, since I do plan on making the 5-level classes the only option in the campaign. I'm hoping that by the time my turn as DM comes up, my players will be willing to try something relatively novel, as we'll have 4 years of standard 5e under our belt at that point. I'll show them some base design concepts in a few months, and see if there are any strong negative feelings then.Yeah, I'm 100% on board with 4-5 level "class sprints" that cover one tier that then end and if you want to get the next level, you've gotta seek out some world element to get it.
The challenge is in the fiction abilities / flags. Being a champion fighter is character-defining. Champion fighters are a certain sort of character and, in addition to their combat prowess, have elements spread out over 20 levels that enhance that "personality." Being a squire-knight-dragonslayer-war king is less one narrative and more four different narratives, and each one has less room to breathe in terms of class abilities. They also don't necessarily play nice together mechanically - one price might be that you'd have to limit novel class mechanics so that they could fit together (no "one class has spellcasting / one class has pact magic" stuff).
I don't think these are insurmountable challenges, but I do think the class system will look very different than the existing 5e class system, and I'm not totally sure they'd play well together (ie, if someone played a Champion Fighter in the same campaign as a squire-knight-dragonslayer-war king, I dunno that would work....), so it might be a situation where you replace the class system in 5e (which is a big part of the game!) with this system (and maybe another system or two)...which makes it a bridge too far for a lot of folks to absorb.
But I'd love to hear about any experiments you do in this avenue.
I'm hoping the fact that the base classes are setting specific, and not merely a redux of the standard classes will help with buy-in. In the past, I've found small mechanical changes almost cause an uncanny valley effect ("It's like a bard, but not quite!") , whereas introducing something completely novel opens up people to rethink their assumptions. Obviously, that's very group dependent, and I'm making some judgments based on my group's own idiosyncrasies.
Also, since the bulk of the progression is based around mastering magical items, rather than joining a particular group and assuming that identity, I'm hoping the character feel won't be so disparate. More of a "Muthin bin Targel is a corsair, a raider of the sea. In his journeys, he has mastered the Sabre of a Thousand Cuts (a sword of sharpness), trained in the techniques of the legendary hermit Ravel al-Nasr, and become a boon companion of the djinn lord Zephylious, and granted mastery over the air (through his ring of elemental command)."
Mechanically, it might work out that he's a corsair 5, a warrior type class with some extra movement abilities, a small pool of superiority dice, some methods of spending them, and Extra Attack as the 5th level capstone. He spends 3 levels mastering the sword of sharpness, increasing the + of the sword by 1, granting himself the Champion Fighter ability to crit on a 19, removing the attunement requirement of the weapon, and gaining the ability to always summon the sword to his side if it is out of his grasp. He completes a test for a hidden hermit who's also a swordmaster, and spends three levels honing the techniques he's been taught, gaining the Mobile feat, a +2 to his Dexterity, and the ability to make a third attack. The ring of elemental command is a powerful item, so he could easily spend 6 or 7 levels learning to use the ring, gaining the ability to use some of the ring's powers more often, and gaining the ability to innately cast some of the spells of the ring.