Pool of Ideas for a Future Game:
Boss Rush 2D Platformer with bullet hell style mechanics (what is the gameplay between bosses? Standard platforming/bullet hell encounters/interacting with the world? Adding smaller enemies seems against the Boss Rush idea. Platforming between areas may seem like content added solely to fluff up the experience. How does Shadow of the Colossus get around this? That is purely a boss rush style game, where silent exploration fills the gaps between encounters. Small things like fruits and lizards are added to either encourage exploration or give you something to look out for while your making your trek. The space itself is interesting enough, with the environments adding to the experience undergone by the player - travelling through a desolate wasteland with almost no other life, save for the beings you are tasked to destroy. The feeling of loneliness and isolation told through the world, supplemented only marginally by the cryptic narrative. It works incredibly well for that type of game, but I doubt that it would work well with the atmosphere meant to be much livelier. Everhood is essentially a boss rush with only minor traversal that is occasionally altered to fit the theme of an upcoming encounter or present atmosphere.)
At present, I am thinking of supplementing the space between encounters with optional narrative and character interactions. Certain items and augments can be found for those who take the time to explore and interact with the world around them.
Interactions with other characters follows a sort of Visual Novel structure; what you say to characters will affect their perception of you and possibly the outcomes or presentation of an encounter.
Core gameplay centers around using weapons to reduce enemy HP, and/or surviving an encounter for long enough to wear out the enemies stamina.
Interactions will change depending on the number of encounters ended through combat versus survival.
Some interactions must be ended through combat (unsure, removing the sense of agency feels counter to the establishing ideas)
NPC where if you ignore them it changes the narrative of the game, or at least their respective substory.
EDIT: October 24, 2023
I completely had forgotten about Cuphead! That game is literally "platformer boss rush" (with the exception of the handful of 2D levels, I suppose). It may very well be worth it to replay it again and study through the level design and enemy attack patterns.