The idea of class niche does not exist anymore when it comes to mechanics. Instead, you have class fantasy and class theme.
Let's look at the druid.
The druid has a spell list of AoE effects (ranger, cleric, wizard), divine magic (cleric, paladin), primal magic (ranger). Most of its features come from its spells, of which there are many. It's core feature, Wild Shape, allows you to become an animal. Not so much a battle beast unless you go moon druid, but a fun little animal that can sneak or swim or w/e. Or you can summon an animal. Either way, animals are involved with the fantasy of the class, but its actual niche mechanically isn't clear. Is it a sneak? A blaster? AoE focused? Tank? Secondline defense? Support?
These things don't matter anymore. All that matters is that you create an interesting fantasy for the class itself, and make a FUN mechanic that captures that fantasy. Viewed through this lens, making new classes is easy. After all, outside of D&D fantasy, many D&D tropes aren't that common anymore, and many very common tropes aren't seen in D&D. So, it merely becomes a matter of seeing something in Fantasy you think is cool -- or having your own ideas -- and then building it out and giving it a FUN mechanic.
You could easily make a full "Shifter" class even though the Druid exists and put it in vanilla D&D without any loss. That's because a full Shifter class would have a lot of non-spell features to modify its "Beast Shift" or whatever, creating a different experience. It doesn't take away from the Druid, who is the wild of the party, and instead lets you either remix that fantasy or pursue a different one (wild barbarian, werebeast, awakened thing, evolution powers, mutant, etc).
So if we look at the Arcane-Gish, we can see literally hundreds, if not thousands of magic-using warriors in Fantasy. Many of these we can group up based on any amount of valid-but-concocted parameters to create a class and subclass suite for.
Reach in Shounen Anime to grab Naruto, Bleach, Hunter X Hunter, Jujutsu Kaisen, and Yu Yu Hakusho and you can create a cool combat-Exorcist that weaves in spells between their attacks. Reach into Western comics for X-Men, literally just the X-Men, and create a hyper-customizable Mutant class whose limited spells also amp their mutations in different ways. Reach into Star Wars and take a variety of sith and jedi and other force powers to create a reinvented Mystic that is actually using a few arcane spells and that has a lot of features about discipline, calmness, and inspiring others. Reach into the Witcher and take each of the Witcher schools to make a Slayer gish class that has different spell schools for each Slayer subclass. Do what I did and draw from Fate/Stay and Arthurian tales to create a Pendragon whose mythical weapon allows them to also cast spells.
Just right there, in a few minutes, I created 5 different valid Arcane Gish classes, showing the flexibility of the archetype AND drawing on popular media, thus demonstrating plausible popularity for these concepts. I didn't even touch Sanderson, Avatar, One Piece, Dragonball, or many real-world myths such as those seen in Indian and norse epics.
The only thing holding us back from getting new and interesting classes that expand D&D and allow us to fill class fantasies like "arcane gish" is the conversative nature of both the designers and the fanbase. It truly is a match made in heaven. Most of you cannot stomach the idea of new official classes because you have 30+ years of lore backing your individual schemas. Additionally, people are so afraid of "bloat" that somehow the new gen of RPers is terrified of it despite not growing up with 3E, as if a DM was never allowed to just pick and choose what content to use in their game.
Alas, third party and homebrew are where the real heart of D&D is IMO, creating new classes, mechanics, and procedures to evolve the game instead of just insisting we all accept the 12 core classes and never dare ask for more.