Hello .Keys!
Two in my opinion really good 'metroidvania' type of games available here on GOG.com--that to my surprise were not mentioned so far--are:
Treasure Adventure Game (and it is free!)
or its remake:
Treasure Adventure World (but I have not played the remake, yet.)
Odallus - The Dark Call "Treasure Adventure Game" is a little rough around the edges (not just graphically), but otherwise a great 'metroidvania' with a focus on exploring the game world and its puzzles, less on fighting. And it does not have an experience-leveling system.
"Odallus - The Dark Call" is my favourite among the new retro-looking 'metroidvania' titles, since it is inspired more by the original Castlevania-experience and atmosphere (Castlevania 1, 3, 4 [Super Castlevania] and "Rondo of Blood). The game world is not 'one' huge interconnected map, but each individual level features lots of freedom to explore and plenty alternative pathways, giving me a very similar experience to the early "Metroid" and 'metroidvania' games. Furthermore, multiple levels have interconnected 'secret' or hidden levels and areas, which on discovery add to environmental story telling in the game. Here is where "Odallus" shines in comparison to many of the so called 'metroidvania' games: the level design, both regarding the platforming/traversal and secrets, as well as the atmosphere!
With each new area you discover, you not only progress with your character's abilities and equipment, but also see the mental state of him progressing or should I say deteriorating--this is depicted by the environments, enemies, sound effects and background music.
By the way, I do not agree with your 'definition' of a 'good', 'pure metroidvania' based on "Castlevania Symphony of the Night".
Neither that "Symphony of the Night" represents the 'ideal metroidvania', that everyone should try to emulate or copy!
First, "Symphony of the Night" itself is plagued by horrible level design with lots of boring corridors and a cheap content/time padding trick of flipping or mirroring the map vertically in the second half. (I do not know which one of these two 'features' were the driving force for the other one, but it is obvious that they would not be able to simply flipp everything around without having to change or adapt a lot, if the map were not constructed of so many empty corridors!
Secondly, I think, that the item or skill progression is not well balanced nor well distributed throughout the game.
Thirdly, the roleplaying elements (experience points and leveling system) feels tagged on and harms both the game balance and the overall pace.
And finally, a really important aspect of the "Metroid" and "Super Metroid" experience is totally missing in all 'metroidvania' games made by Igarashi or those copying just his formular: the loneliness or solitude of the protagonist factoring in not only on the mysterious atmosphere of slowly discovering layer by layer through expanding exploration, but also on the triumphant sensation of overcoming bigger and bigger challenges or odds during the course of the adventure.
Both are replaced and tarnished by a taIkative cast or population almost guiding or at least obviously directing the player to where he should go next, and by poorly implemented roleplaying-like growth systems.
To me it seems that Igarashi only took the map system, progress gated by acquiring corresponding items or character skills, and the teasing of a well equipped character in the beginning only to half-heartedly taking away everything from "Super Metroid".
Maybe those "Symphony of the Night"-like games should be called "Igavania" or "Symphonyvania" instead, since they actually have very little in common with both the original "Metroid" and "Castlevania" games.
Independent from how to call these types of games, I found out, that I do enjoy the more creative interpretations of a 'metroidvania' that focus on fewer or just one aspect, such as "VVVVVV", "Yoku's Island Express", "Ori and the Blind Forest" and "Odallus The Dark Call", much more than the 'jack-of-all-trades' Igarashi 'imitators' and 'copies'.
Kind regards,
foxgog