Because the Metroidvania genre turns on gaining access to new areas as you acquire new tools, it stands to reason that a fair amount of the designer's attention should be focused on creating a variety of useful tools. And, because Seq.Breaker is about getting by with as few tools as possible, these tools have to be extra useful-- they have to be constructed with certain "unintentional" properties that the player can exploit in order to get to places they "shouldn't".
My current estimate is that there are going to be fifteen or sixteen different items in Seq.Breaker. If that sounds like a lot, consider that they're going to be distributed amongst the game's three missions. That is, at the start of each mission you have zero items at your disposal and in each of the game's missions you have access to a different set of tools.
These tools don't overlap at all. The first mission gives you access to a shield power-up, a standard gun, a laser gun, and bombs. This mission is very action-oriented, hence the preponderance of weapons (though it should be noted that someone breaking the sequence can complete the stage without killing a single enemy). The second mission is much more navigation-focused: there's scuba gear that allows you to spend more time underwater, an air jump power-up that allows you to reach higher and farther platforms, a "shelf gun" that appends quickly-disappearing platforms to the sides of walls-- and so-on. In total, there'll be six tools for the second mission, and only one of them-- acquired just before the game's first proper boss battle-- is a weapon. With almost no offensive capabilities and only one hit point, this second mission is going to be quite a challenge (especially if you're following the sequence).
For the third mission-- which, as I've said before, is about the size of your average freeware Metroidvania game all on its lonesome, with several bosses and branching paths and so-on-- I'm going to be giving the player five or six brand new tools.
So, each level has its own enemies and items, with absolutely no overlap in those two categories. In a way, they each feel like separate, individual games; I like that. It puts the emphasis on the connection factor, on the big idea, on breaking the sequence.