Much of the art will be dependent on what the artist(s) that join are capable of, but here's a general overview:
Aiming for realism, the art will be there to immerse the player in the world. Hyper-stylized and extreme or anime proportions can distract from that. The art style can be recognizable, but it should be clear and with realistic proportions.
The detail level and medium are open ended and up for suggestions. 2D line art is the current plan, but pixel art and 3D isn't off the table if an artist that prefers that medium and can make it match the feel of the game joins the team.
There will be some animations and gifs, but the complexities of them are open for discussion.
The UI will need to be clean and easy to understand at a single glance, but have enough of an aesthetic appeal that it doesn't detract from immersion.
All of the visuals will need to be cohesive enough that it doesn't look stickers on a background.
Below will break it down even further:
Starting with sketches and crude drawings just to get a temporary asset is totally acceptable and the progression to getting the final style will be an ugly one. So as long as our final goal is the same, it can be as ugly and unfinished as it needs to be in the meantime.
The sections below will focus on the final goal, or ideal. If these are unreasonable, or if there are better ways to go about them, please express this.
It should feel approachable and intuitive with a style that blends well with the other visuals. While the buttons should be obvious, they shouldn't be distracting.
The UI will have some non-text cues for the events of the game as well, such as glowing outlines on stat bars, scrolling text, tooltips, and more. It will be what the player will be interacting with the most.
Ideally, the appearance will change based on where the pc is to add to immersion, and or potentially be customizable.
With the game being very nature heavy, there will be a lot of nature scenes as well as some unique urban ones in places such as caves, underground, desert, and so on. Each town will use building materials that suit their environment, but none will be wood, so no log cabins.