What I would like:
Coherent style within books. Each book should have a definite style, consistent across all of its art. It doesn't all have to be by the same artist, but it should at least look as if the artists went to the same school.
If two books are closely connected--typically by setting, such as Van Richten's Guide to Ravenloft and Curse of Strahd--they should have the same or similar styles. However, Wizards doesn't do 2E-style product lines any more, so for the most part each book will stand on its own.
Distinct styles between books. You should be able to look at any given art piece and say "That must have come from Tasha's Cauldron of Everything," just by the artistic style alone. Every time you pick up a new D&D book, it should be a moment of discovery to see what they did with the art*. To hell with brand identity; the identity of the D&D brand is bold variety. (Yeah, I realize that's a tough pitch to the marketing department.)
The Core Style. So if every book has its own style, what should the core three be like? I was going to suggest a melange of different artists and styles, but on reflection, I don't think that would work. It would just look like a mess.
Instead, what I would want (and this is just my own personal preference) would be "realism-plus." Everything should look
close to the real world, but just a bit more dramatic and theatrical. Armor is well designed and functional, but with more baroque and decorative features than you'd expect on a real battlefield. Swords are properly sized, but also a bit decorative, and they gleam as if they were just polished. People are heroically but reasonably proportioned, and they are either unusually clean (for adventurers slogging through a dungeon) or fantastically begrimed and bloodied.
Style-wise, I'd want intricate detail and crisp edges with minimal blur. Subdued and diffuse lighting for the most part, not a lot of intense point sources. Inline art should have no background at all. For full-page art, the background should also be highly detailed, and it should have that same "realistic-plus" look. The landscape should look like something you could imagine seeing in the real world--but if you did see it, you'd stop and stare in awe before clawing for your phone to snap a picture.
Basically, I want Larry Elmore back, minus the chainmail bikinis and the '80s hair. Well, maybe just a
little '80s hair.
*Okay, it won't really be a moment of discovery, since you've undoubtedly seen the cover and the cover should be an exemplar of the style. But if you hadn't been keeping up on the latest product and someone gave you a wrapped copy of the book as a present or something, it would be a discovery.