J-Snake: In that case, you should check out Metal Warriors (SNES), I think that captures a little more of the vibe you're going for. And yes, it does include a PvP versus mode, and a more exploration-based story/campaign mode. Pretty terrific for it's time.
Rethinking Metroid gave us Super Metroid. Rethinking Super Metroid gave us crapticles.
Conceptually, platforming PGC is more about creating a sense of flow and direction, and successful Metroiding is about corralling appropriate abilities in appropriate locations. Here's where design can get dicey, because there's a few ways to go about it, depending on your concept of what a Metroidvania is like.
Metroid-style: In this case, you'll want to produce an area for each item, based around that item. The item location is something the player can always reach with fundamental abilities (the rest of the area is designed for player failure without the item), and provides a distinct type of challenge to it. (Clearly, this will scale with complimentary ability sequences; you aren't going to use Super Missiles for the sake of scoring basic Missiles.)
So you stumble around a new area you can't totally circumnavigate until you've acquired what you need to do so (right around the corner), which then has a simple feedback loop/escape point from the item location, a successful reattempt with your new ability, and that sets the stage for other, later sections.
tl;dr:
New Area (Failed Navigation) > Item > Successful Navigation (repeat)
Castlevania-style: Kind of similar, but instead of acquiring an object in one area to navigate around in that area, you acquire each power in sequencial order, so you can navigate around a different area to acquire the next. It's a little more indirect, but just as effective.
tl;dr:
Item > New Area > Successful Navigation (repeat)
Then comes the item flow. While some cross boundaries in-between, for the most part items come down to two types: Navigation Items and Combat Items. This is an important factor in preventing staleness in game of this variety; as it lets you weave back and forth between them, rather than embracing one at the expense of the other.
The areas are also set up to alternate/compliment the focus as well, which is precisely *why* people can play the well-designed ones for hours on end; and why more of the later, failed titles (which weigh too heavily on one or the other) feel all grindy and monotonous - because basically, they are.
Circle of the Moon and Harmony of Dissonance are particularly combat-focused, so navigation in them is boring and feels like a chore; and Zero Mission is almost totally navigation-puzzle-focused (and lacking in combat), which is why games like Super Metroid and Symphony of the Night are so much better than either one. Their lopsided focus on one aspect over the other not only leaves a void in the opposite's wake, but the excessive focus on their strong aspect also makes them more monotonous.
Mostly used those because they're the more recognizable examples, not the worst offenders. I'd say those go to each of the series' respective "II"s... sadly another consequence of rethinking? Ironically, it's Met II's combat-focus and CV II's navigation-focus that fails them both. Makes me wonder what you'd get if you put the two teams together...EDIT:
What I am talking about is control and repercussions in every game-frame, taking your environment and equipment into account. That is a new level of action-quality, a new amazing way to think about combat itself. My system exploits the properties of the environment and your tech-toys. This adds a world of depth in itself waiting to be explored.
Sorry to burst your bubble here, J; but this is present-day brown-shooter central.
You're basically trying to pioneer Modern Warfare for SNES. (Sadly? Although I guess there's plenty you can learn from it though, like how players learn to use your weapons/combinations. Still, it sounds more like a glorified psychological beta test than an actual game.)