One of my concepts is a character that wears plate mail but has expertise in stealth, uses both a wand and a sword, does bonus damage when striking from hiding, and can cast fireballs, heals, and resurrections while tracking his favored enemy. Call it a 'concept that isn't modelled well with existing classes' if you like, but I think it's trying to be too many things at the same time. 5e doesn't model that well because it doesn't want to. It doesn't think any one character should get more than a couple of gimmicks.
I don't call that "concept" anything, it's not a concept, it's a collection of powers from a particular game system you want. It's not even close to a concept.
But if you had an actual concept that was able to do those things it would be a fine concept. It would also be a powerful concept so couldn't be realized as a low level character, any more then a character who just "cast resurrections" could be created as a low level character.
Considering that there's opportunity costs while leveling - you get to do X or Y, but not both, you could probably realize this at a level when others are casting wish, making you rather low powered because you spread yourself in so many different directions.
But you're right, 5e doesn't model it well - which does not make it a bad concept, it makes 5e a bad system to try to realize that concept.
Also, a concept doesn't "I have the favored enemy feature" or "the expertise feature for stealth", it's has "I hate hobgoblins and are good at hunting them" or "I am very good at sneaking". "Favored enemy" and "Expertise" is one of several mechanical ways of modelling that, and in one system only. A concept has nothing to do with system. I can attempt to find mechanics to model the same concept regardless if it's D&D 5e, PathFinder, FATE, Rolemaster, or Dungeon World. Some will be better or worse at modelling it.
This is a concept:
"I am Morak Twinember. Ever since the raid by hobgoblins on our smithy killed my pa, I've hated them. I quit the militia when they wouldn't go after them and tried to deal with them myself, tracking them down and sneaking into their camps. But my eyes were opened when I met Master Wen, the Hermit of the Spire. He awakened the magic of the Sun God within me, as well as a sense of larger purpose."
This is a mechanical realization of that concept in a particular game system:
Mountain Dwarf Ranger 2/Rogue 1/Sorcerer (Divine Soul) 11
Heavily Armored feat.
Favored Enemy: Goblins and Hobgoblins
Expertise in Stealth and Tracking
Sneak Attack: +1d6
Can pick from the sorcerer and cleric spell lists. Has 5th level spells. Among spells known are Fireball, Heal, and Raise Dead.
If you want more sneak attack, feel free to increase rogue level - this is a high level concept. Again, at the opportunity cost of not advancing other things.
But I could just as easily of taken that concept and modeled it in another system. Concept is independent of system. What you had was a wish list of abilities.