Spellcasters! They are everywhere.
Unless you run a straight barbarian and never multiclass you will get the option to become some degree of spellchucker in 5th edition. One thing I liked about 3.x was that an non-spellcaster had to chose some pretty specific things and develop in a specific way to join a prestige class (at the DMs approval) to become a spellcaster.
Now everyone can chose a class option and become a spellcaster and even the barbarian if he wants to can take the Mage Initiate feat and gain a some magic to throw around.
It is certainly not game breaking by any means but it is telling that every single new class that has been added since DnD was created way back when is some sort of spellcaster. Maybe it is just the easy way out compared to developing interesting and effective non-spell abilities in sufficient quantities for more non-magic classes.
Maybe a better way to state the question is:
What's the design space for new primarily "martial/non-caster" classes? Are there "martial/non-caster" concepts players have that current classes aren't meeting?
For instance, Mike Mearls once worked on
Iron Heroes which created a whole bunch of "martial" classes for that game. It can be done, and one of the lead D&D designers has done. The question is would it add value to the game? Is it something we as players want? Does it fill a unique design space?
My observation/opinion is that WotC knows how to design an excellent "martial" class – the Rogue is, in my mind, the epitome of how to do it well. The Monk and Barbarian are also well done, albeit with minor flaws that are being discussed. And the Fighter's design is the most mediocre (or, a kinder way to say it: most
conservatively designed) of the bunch. I think this harkens back to an ongoing shift throughout the history of the game that assumes "players find magic more interesting, and fighters are plenty popular, so we're doing fine with an abundance of casters."
My hunch is there's a bit of a chicken-and-an-egg scenario happening with this line of thought. Similar to "our statistics find very few players play high-level, therefor we don't produce much high-level content." Whereas I tend to think it's a grayer situation, where that Field of Dreams quote applies: "If you build it (well), they will come."
We've seen the Warlock (4e) and Sorcerer (3e) get added to the list of base classes over the history of D&D. I didn't realize I ever wanted to play those concepts or that there was a void in the class design space...but they seemed to be pretty popular among players. It would be interesting to dream up, say, two "martial/non-caster" concepts that might be introduced to the game, similar to how Warlock and Sorcerer were added over the years...
If you look back through the history of D&D's alternative classes, there are a four recurring themes that might provide inspiration:
- Sages/Experts began in AD&D's Sages and Specialists which allowed you to play engineers, scribes, etc. Sounds odd for an adventurer, but I believe the concept was picked up in 3e as the Expert NPC class, and various Dragon magazine articles has "generic adventurer"/"professional" classes that followed a similar theme. Maybe the 3e Factotum class could serve as inspiration.
- Merchants/Tradesmen-type classes have appeared in several AD&D supplements, including Birthright Campaign Setting (Guilder), Dark Sun Campaign Setting (Trader), and Masque of the Red Death (Tradesman). This concept was kind of carried into 3e with the Expert NPC class as well, and possibly with 3e Dragonlance's Noble class, though I'm not sure how it would work given 5e's adventuring class structure & fast-and-loose economy.
- Knights appeared in AD&D's Dragonlance Adventures, there was a Pious Knight class in Legends & Lore, and it graduated to a Knight class in 3e's Player's Handbook 2. The question would be How is a knight different from a fighter in 5e? How does it justify more design space than a martial archetype like XGtE's Cavalier? And if you focus on the 3e Knight's Challenge as being a key class feature, how does that compare to the 5e Oath of the Crown Paladin's Channel Divinity: Champion Challenge?
- Warlords were introduced in 3e's Miniatures Handbook as the Marshal and later in the 4e Player's Handbook; apparently they were so controversial during closed playtesting of D&D Next that the team decided to scrap the concept and put pieces of it into the Battle Master martial archetype. Definitely they have as much history as the Sorcerer or Warlock, so I think it's more a matter of coming up with a design that has broader appeal.
Personally, I think you could merge #1 and #2 into an interesting (if hard to design) class, and you could definitely merge #3 and #4 into a cohesive class.