I don't see anything wrong with a "Castlevania Complete Works" style all encompassing art book.
I'd want to see a COMPLETE complete works of CV art. No BS. I hate when companies release complete works of various games, but don't release ALL of the art in those works, then later down the line, they release a new version of the complete works with MORE "previously unreleased" designs(like alternate designs and unreleased sketches). None of that, "Oh, there's a few things we didn't include that's in the CV vault!". EVERYTHING! Every monster, every character, every item, background concept art(even for the older games, as I'm sure it exists), the whole deal! Square is pretty good a BS-ing with art. It wasn't until the release of Amano's Sky artbook collection that MOST of his FF work was actually allowed to be shown to the public, especially designs that most people never even knew existed. I still hope, one day, they release an "Complete Collection" artbook of FFVII, which includes all art, even for minor NPCs(Mr. Coates, Ester, Dio, Johnny), scrapped ideas(female Sephiroth, Jenova's cloaked body parts, Cobalt and Indigo) and Nomura's pre-production art for Sakaguchi's original characters(Detective Joe, Nicholas the Needle,...).