I like old-school artwork AND the new stuff.
Part of the reason I liked old school artwork so much was the variety. Artwork by Elmore, Erol Otus, and Dragon-Mirth style cartoons all existed in the same books.
I would love to see that again.
I am 100% with Mattachine on this.
I love the "realisitc snapshot" looking stuff of Elmore, the hazy-edged fantasy-"not quite real" look of Easley, the clear but almost impressionistic/Van Gogh-esque of Otus, the ink-line drawings and humorous cartoons, and the "near-comic book" dramatic character looks and fight scenes of later books.
IMHO, 5e ought to have samples and examples of every and all! The game is supposed to evoke imagination and creativity, so show some...a diverse amount of some. Having a single artist's/the same style throughout detracts from that...for me.
Also, LANDSCAPES & SCENERY...background in general! Not every image has to be a close-up or even "mid-range" shot of the characters. Show a group of tiny adventurers in the corner of some HUGE cavern or sprawling vista with monsters charging (or not even noticing them) from afar. Make me wonder, what else is behind those trees, around that cavern wall, or over that hill?
One of the most evocative image/series of images for me, is from the 1e DMG. Those 4 pages or so, showing a group of really poorly drawn, but still distinct, adventurers entering the "dungeon", moving from place to place encountering different creatures and eventually finding that tiny passage that ended in a great treasure trove.
It was an illustration, pure and simple. What was that female magic-user casting on the Stone Giants? Was that elf going to hit those Salamanders with his bow again? HOW did they get through that chamber with all of the trolls?! How were they going to get all of that treasure out again when only the dwarf and halfling were going to be small enough to get in there?
That's the art I want to see. Make me wonder again. Make me imagine.
(and PS: If you want/need any "simpler/older" style drawings, let me know.

I don't "draw" on a PC. It's all by hand.)
--Steel Dragons