Hussar
Legend
You never allow new classes?What's good for the goose is always good for the gander. A monster might very well have unusual powers that PCs never will, but if an NPC has a class and levels, then it plays by the same rules as the PCs. The NPC might be able to do something extraordinary via an unusual spell or a rare magical item, but in that case, any PC who obtains that spell or item can thereafter accomplish the same extraordinary thing.
Your group only has a limited number of players, so, presumably, a limited number of classes in that group. Are NPC's limited to those classes? Of course not. They can be classes that the players didn't choose as well as potentially being classes that the players didn't even know about since they didn't exist at the time.
Meh, all my baddies are variants of Binders. They have weird powers because of the Vestige's powering their powers. You can't copy them because you have no idea what Vestige they bound and you aren't a binder. What? Binder's don'T exist in 5e? Well, maybe they do.
I have a really hard time believing that this is actually a player side issue. This is something that people invent to try to brow beat other players with their badwrongfun. "Oh, you MUST have explanations for all this" - we do. It's pure gamism. "Oh, that explanation isn't good enough and WotC is intellectually bankrupt for never giving us a single explanation that I can then use to brow beat my players with!"
Yeah, no thanks.