I don't really know where to start when it comes to the art for my game, help

I'm making a 2d metroidvania with 3 of my friends, originally the game started out as a game jam game but we all collectively agreed we really wanted to continue working on it, and make it a fully fledged game. I'm the artist, i do all the animation and design work while the three of them focus on the programming part of the game.

We're currently at a stage where we're a little stuck, we have a lot of the basics we need, visually and code wise but i'm having some issues with the art direction, i'm not particularly happy with the overall vibe i started going with when we first started working on the game for the game jam and now that we're committed to it i really want to make it the best it can be so i'm doing a slight rework, at least going forward and i'm not sure where to start.

I want to make sure i stay in the concept art stage more than i usually would for my solo projects because i wanna make sure the designs i end up going with are actually good and something we're all happy with, because working on such a big project it's gonna be a nightmare to have to go back and redraw and remake everything i'm not happy with which would eventually happen, i wanna try and avoid that as much as possible right.

But i don't know where to start? There's just so much that i need to do i'm not quite sure where to start, we have the main character designed, basic concepts for some of the areas and things, an outline for the story, the very basic things, and i have a pretty good idea of what i want the style of the game to be i just don't know how to make it a reality.

This is really poorly worded but i hope i got my point across, what would be the best place to start, in terms of concept art at least?