* movement
* shooting and killing enemies
* non-shooting enemies
* items
* items menu
* physics
* changing 'state' of player
But I want to speak about Metroidvanias in general, my amateur take on the genre and what I deem to be the greatest problem I am facing - stamina.
Division By Zero will be a Metroidvania. But was does that even mean? Sometimes it seems like Metroidvania is used for all advanced (or rather complex) side-scrolling platformers. What I kind of like the more narrow definition some people suggest.
An important characteristic of a Metroidvania is openness, an interconnected world. But this needs to be narrowed down. A Metroidvania is not an RPG, I think. It borrows elements from the RPG genre, but it's not open in the same sense. To me, a good RPG is possible to play pretty much regardless of the main story or plot. If you choose to, you stay in a world and randomly walk around, discover, explore, hunt (or get hunted) by monsters. It's not the objective - the intention, especially not from the game-makers' point of view - but it's possible. Surely this is possible also in quite a few Metroidvanias? Yes, it is. But not the same degree, I would say.
A Metroidvania is not fully open, but in comparison to a level-based side-scroller platformer such as the Super Mario-series, it is. You often have to back-track to move forward. And this would not be possible - no one would stand it, play it - if you did not get incitements, such as progress in the form of a new ability or some might weapon, etc. But in a way, you could say it's the same thing.
The main difference is that in a classic platformer you have no illusions about being forced to walk a path (you know you are), while a Metroidvania often includes the possibility of many paths. However, this quality may or not be illusory. And in the end, they're just so many paths leading to an objective. The main difference is that the player is lured to feel free. It's a magic trick. You and the player know this to be, but the game is much more enjoyable if the player forgets about it and you make a world that is optimized for forgetting it.
While an RPG has an open world (or at least a pseudo-open world), a side-scroller has levels and Metroidvania has a perceived open world (with 'hidden', implicit levels).
Division By Zero is my first serious game and I am counting on failing in some way when I'll build the world. In this, my greatest enemy will be myself. I enjoy programming, I love building things. But in relation to building things using programming, I don't enjoy making levels and worlds as much. Building levels is just as hard, have other difficulties and different joys and I guess what we like is very individual. Oh, it would be nice to be part of a team!
...While I endure? I think so. When I've made before I haven't been very interested in the games in themselves, only to try some game development technique, play around with a new language or the sort. But this time I feel the game is interesting, a game I would want to play. Division By Zero will not be the best of Metroidvanias (not even close), but I believe it will be enjoyable. And a game I would want to play. If I meet this objective, it will have been worth the effort. If more people will want to play it and actually pay a few bucks to play it, I would be very proud and happy.