I'm probably conservative in 5e when it comes to classes (especially as compares with 3e and 4e). Part of the reason for this is that more classes means steeper barriers to entry for the game. It's splat-bait, it's easy to add choices without adding substance and relevance. The more classes there are, the less chance any one of 'em sees actual use at the table, and a class that doesn't appear at the table is a lot of wasted time, effort, development dollars, and page count. In my mind, a class needs to pass a pretty high barrier to be "worthy" of its mental and physical and temporal space.
One of the most important barriers is that each new class needs a unique "story." That is, there's a unique place it gets power from, a unique use it puts that power to, and a unique goal that is part of the class's DNA. The current crop of 5e classes does that pretty well. You can put a Sorcerer next to a Wizard next to a Bard and they all have different stories to tell - stories about magic woven into your being, stories about knowledge conquering the unknown darkness, stories about brave, heroic, charismatic wanderers with a knack for spellcraft. That's all "just fluff," but a lot of proposed classes fail on that fluff requirement. When someone proposes a Swordmage as a unique class, for instance, I'm like, "Wait. I get that 4e swordmages had some fun powers, but did they have a fun story? Were they unique in where they came from? Were they unique in what they did or what they sought out?" Not really, IMXP. For the swordmages I saw, the main appeal was not the story of being a swordmage and what that represented in the world, but rather than mechanical delight of "heavy armor + arcane magic."
That mechanical delight is nice, but it isn't big enough in my mind to be a class. It'd go good as a subclass - like, the EK subclass. The "cool powers" are nice, and they can be spells. And no class was needed.
Something like psionics, or an artificer, may or may not pass that important threshold, depending on your perspective, but a lot of proposed classes can't achieve that initial escape velocity.
There's other important points too (psionics and artificers specifically, even if they achieve story distinction, struggle with being mechanically separate and having similar play experiences to other classes), but story is probably the first and most important.