I'd love to have another Metroid-vania game. However, if it were made, there's one particular game design aspect I'd prefer it to avoid: arbitrary limitation of exploration.

The sub-genre has always irritated me, to an extent, because of how you can only explore as far as the game will allow you to based on what avatar upgrades you've accumulated, as opposed to based on your own skill level. Not going to count sequence-breaking, partially because it's not always intended by the designers, and partially because it only becomes relevant after several playthroughs and a higher level of devotion than the typical player is willing to give.

Like, if I see something that I want as I'm exploring, I want it. Right there, right then. If it's something important or valuable, make it tougher to obtain...at least until the player has become more accustomed to the overall gameplay mechanics. Don't artificially limit my access by forcing me to obtain the double jump orb, the beam upgrade that just so happens to shoot through walls, the suit that grants me a longer underwater time limit, etc. Now, I wouldn't necessarily be opposed to upgrades that make it easier to explore the world and access hard-to-reach areas, but don't make those upgrades mandatory.

Though not a Metroid-vania game by any means, I really enjoyed the skill-based progression in the sidescroller Dustforce. First off, there are no avatar buffs. You begin with every ability at your disposal, and the tutorial covers the essential and intermediate mechanics, but only through extended gameplay will you discover the more advanced mechanics.

Second, in regards to the world itself, it's a nexus that separates into four smaller nexus (I believe that's the correct plural form, strangely enough >.>), and within each of those are 12 levels. All of the sub-nexus are accessible from the get-go, along with the 4 easiest levels within each sub-nexus.

Now, TECHNICALLY the other 8 levels (4 of moderate difficulty and 4 of hard difficulty) within each sub-nexus are arbitrarily locked until you've gotten keys, which are obtained by getting high ranks in levels of the next-lowest difficulty class. That said, it could be argued that it's not really an issue since you need to familiarize and learn about certain gameplay mechanics in the easier levels in order to beat the more difficult ones.

Anyway, what really fascinated me about the world was the placement of the levels within each sub-nexus: the higher the difficulty class, the tougher it is to actually get to the level from the sub-nexus. In fact, it wasn't until hours of gameplay later that I realized it was even possible to traverse certain areas of each sub-nexus, and more often than not I was rewarded with the location of another hard-class level upon a successful traversal.

...and wow. Just noticed that I turned this into a complete rant. Ah, well. Hopefully this can spark some insightful discussion. Hopefully, I made this understandable. <.<