If it's not yet finished, it's too early for it to be in the PJ gallery xP

As for textures - I suspect working with a downsampled drawing might actually make things a bit more difficult, since it's tempting to follow what's there, when what you should probably do is think about the textures "anew".

There are lots of ways to approach detailing a pixel art scene. One way you could try is to get rid of any bits that are only 1px wide or tall (so, any stray pixels, as well as any very thin features other than the rimlights that you're confident in). Then, clean up your larger shapes so they read well and look clean. After that, add new pixel-level details (small branches, bark texture,  etc). When it comes to how these details should look, again there are lots of ways, depending on the style you want. If you're at a complete loss, you can always check out how other artists have done similar things in their art.

Yeah, there are lots of unfinished spots. It's somewhat difficult to figure out what to do in large textured spots like the bark or the ground, are there useful ways to think about that?

Thanks anyway! I'll see if I can finish this :)

Overall this piece looks rough and unfinished, as if you  took your older artwork, downsampled it to a smaller number of colours, but then didn't take the time to go over it and give it pixel-level polish outside of a few spots. Where you did polish things up, it looks good! You should give the rest of it that same polish. As-is, the piece looks noisy because without polish, its pixels don't read like anything.