Warning : I could be misinterpreting the historical development of the genre and be 100% wrong on this, I don't care. Do your own research & write your own article, bitch! (but also link it here if you do so I can learn, bitch)
If you've played the recent Ghosts N Goblins Resurrection (if you haven't, do it, legitimately amazing game that might be your game design Canon Event) then you might have noticed that it has a very unique style of gameplay that you won't find in any other major platformer series around. The man himself, Matthewmatosis certainly felt that way.
However, even though GnGR is unique today, that wasn't always the case. GnG as a whole is a product of its era and end environment, and a reflection of a broader trend. During the 80s the arcades had a small "movement" of sorts - a bunch of action platformers that had a similar sort of chaotic, rng and improv heavy gameplay such as Legend of Kage, Ninja Spirit, Ninja Warriors, etc. Chaosvanias, if you will. Even games that were later ported to console such as Bionic Commando had this feel.
These games were inspired by Spartan X and Green Beret's style of design (which developed into/side by side with run n guns and beat em ups) - they were a lot flatter, they didn't focus as much on terrain or tightly arranged traps. Instead they focused on flooding the player with constant, sometimes dynamic enemies that would either come from the sides or spawn at random locations, giving the games a very chaotic action feel that had more in common with beat em ups at the time (again, Ninja Warriors) than modern day action platformers. There would be a lot of unpredictability run to run, randomized elements overlapping with static elements forcing you to adjust your routes, and some bullshit as the cherry on top.
However, once we moved into the console era, the limitations of the system forced developers to move away from this style of design because they could not handle the chaos. Terrain was a lot softer on the hardware, and as a result the game design shifted towards it. The only subgenre that preserved this style of design is run n guns, but even that is slowly getting lost in favor of more static challenges (and fucking boss fights).
This Megaman interview describes the process :
There was one difficulty with Matsushita’s programming, though: his code was slow. Being so perfectly wrought, with lots of internal safety checks and the like, meant that it took a lot of processing power and ran slowly. His routines for hit detection, for example, were very slow. My first idea for Mega Man was actually something closer to a STG game, where there’d be lots of enemies on-screen at once and you’d have fun blasting them all. But due to those programming limitations, I had to change the type of game.
The new game I envisioned had fewer enemies, and more varied terrain to obviate the need for multiple enemies on screen. One unexpected benefit of this new design was that less enemies meant we had the resources to do that cool layering effect on Mega Man’s character sprite. Without Matsushita’s programming abilities, though, I don’t think we would have even attempted it. It’s not something we normally did.
The resulting game design drifted more towards a static, puzzle-like style. The games started using enemies not as active threats but more as obstacles with specific solutions, and ditched a lot of the random chaos. You can see this in Bionic Commando's NES port - the design is a lot more solid and polished, but it feels a lot more "dead" and static due to how the enemies move, or rather don't move.
This style of design has become synonymous with platformers, and even built up our understanding of "good game design" as a whole. But is this really the peak of the genre? I don't think so, I think we've reached the dead end for that style of design, and it's time to EMBRACE CHAOS!