See... THIS is some clever game design! You mind if I use this?
Which means making subclasses as bland and non-interactive with class features as is possible in order to ensure they're not bound to a given class... which I personally find bad design.
Like... If I make a Rogue Subclass that does not interact with -any- Rogue abilities whatsoever to make it available to ranger, bard, fighter, and other classes, it's just gonna feel one dimensional AF. 'Cause it can't interact with any of those class's features, either.
But it'd work for Bard/Cleric/Druid/Sorcerer/Warlock/Wizard because you can use spell-stuff since they all get it. MAYBE Paladin/Ranger, too.
Like the Life Domain wouldn't work cross-class because Wizards don't get Channel Divinity for the 6th level Preserve Life subclass feature, but the REST of the subclass would work for other caster classes.
Meanwhile if I did that in A5e I could at least make Martial cross-class features interact on Combat Traditions... but it'd still be pretty dull, all things considered.
Damn... now I kind of want to try a cross-class martial archetype for Martial Artistry... I feel like it could maybe work so long as the archetype granted and played off it's own features and improved or granted maneuvers/traditions/exertion...