Figure out the art direction/guidelines
Let's get the conversation started on the art direction. The keywords are simplicity and clarity: the art shouldn't take too long to create, update or extend. And it should be consistent.
We got started this morning with @guilhermehto, as he's working on UI tutorials :

Just updated, organized the UI kit to create some kind of baseline for the project:
- Use visuals to explain how the assets work as much as possible

- When a design was approved, group and name layers so it's easier to edit and improve moving forward - it's important to make it easier for other artists to join the project.

- use folders as categories so you don't have to repeat the asset's name in every sprite at export time (I've seen to many examples like
Wall/Concrete/Wall_Concrete_128px_Left_Edge.png->128w/wall/concrete/left_edge.png)
That's for organization. Then I think it's best to build as we go and iterate over the design. We'll be using the assets we make at gdquest in tutorials so we can see if everything works well in Godot (and share the Godot projects back). We should check that the designs aren't too hard to work with in the engine - thus keep things simple. The UI above should work as nine-patches no problem.
Here's the interface in Inkscape format: