Recently, Karubiimunomono's posted a few settei (concept drawings) from the Japanese LGPE guide.

https://twitter.com/karubiimunohito/status/1068758164659552256

While most of the arts seem to be the same ones that we can find in the Western guide (scanned by PokeKalos in the link below), some of them are new (e.g. capeless Lance, settei for Meltan & Melmetal).

https://drive.google.com/drive/folders/1dM39LtVSUYHqfrHfOpkpd0DmZRfkpLE8

There's also the preorder Japanese-only artbook, which nobody has scanned, even partially.

In short,
What we have:
-Artwork from the official website (non-concept arts for Players, Rival, Oak, Brock, Misty, Surge, Erika, Elite Four)
-Non-concept arts for Koga, Sabrina and Blaine (but they're just lowres official video cropouts with color/quality issues)
-All concept arts from the western LGPE strategy guide (scanned by PokeKalos, linked above)

What we don't have:
-Non-concept arts for Giovanni, Red, Blue and Mina (these are confirmed to exist) and possibly some others (probably Green, maybe also Bill, Archer, Jessie & James?). They may be in strategy guides.
-Better versions of non-concept Koga, Sabrina & Blaine (the current video cropouts suck)
-Everything from the Japanese-only preorder art book
-All concept arts from the Japanese strategy guide that aren't in the western guide

I don't think the official site will update anymore, so if we don't find and scan the remaining arts, we may never get them.