If you’ve been paying attention, you might have noticed that the newer Pokémon middle-evos have themselves evolved over the years. They used to be this a tadpole state between the starter Pokémon and their final evolutions. Now, the middle evolution of each starter has become a sort of awkward teen phase before they grow up to become true adults.
Don’t believe me? Take a look at this starter evolutionary line for the original Grass-type, Bulbasaur, which includes Ivysaur and finally Venusaur. It’s pretty much a straight line from bud to fully-grown flower lizard-thing.
It's much the same story for Charmeleon, Combusken, Monferno, Servile, Croconaw, Marshtomp, and more. A few of them feel unique, such as Wartortle, Dewott, and Grovyle, but by and large, the middle evolutions used to just be a mix of features from the pre-evolutionary form and the final evolutionary form.
Then, things started to change at around the 6th or 7th-Gen games. By the time Pokémon X & Y rolled around, the middle evolutions weren't so much an in-between tadpole but were instead their own distinct awkward teen phase, each with their own stereotypical trope as inspiration.
Let's start with Pignite, the middle-evo of Tepig. Obviously, Pignite is the overweight kid in high school that gets into wrestling to take advantage of his bulk. By the time it evolves into Emboar, it's become a pro wrestler.
Now, let's tackle Braixen, who clearly never grew out of that "magic is real" phase and re-reads the entire Harry Potter series religiously.
Frogadier? Clearly the kid who wished he was a ninja, but started to get really into the whole Japanese mystique and even went to ninja camp that one year. Frogadier loves Naruto, but not nearly as much as its rival, Raboot.
Raboot REALLY loves Naruto, but eventually grows up, realizes anime was a phase, and gets really into soccer (I'm sorry -- I mean "football").
Drizzile is definitely that awkward emo kid that can't stop listening to My Chemical Romance on repeat, while Dartrix got into the "special herbs" way before any legal state would allow such a thing.
And finally, Thwackey is that drummer kid that will either move onto a successful music career after dropping out of school or become a bouncer at the local bar.
You can expect future middle-evos to be along these same lines, with the next-gen Water-type being a nerd-type that becomes the next Elon Musk or something.