Hey Tortoise, I like that guy you linked to. Nice stuff.
I'll simply parrot what a lot of people have been saying. More diorama type scenes with something going on and a lot less portrait pictures.
One point I would disagree with is the Cantina art. I LIKE the mixed race (as in D&D races, not real world) pics like the Lockwood barfight scene leading up to 3e. Very cool. Hey, it reflects how most groups are going to look anyway. Why should the art be all this or the other race when the average group is going to include three or four different races? Why shouldn't an elven court scene have a smattering of "minority" races in the gallery? It's not like it's totally beyond belief.
And, it's interesting, at least to me. Just what is that well dressed orc doing in the gallery at the Elven court.
But, all that aside, the one, biggest thing that I want to see in 5e art is a return to whimsy. Look, it's fantasy. Go weird. Erol Otus, lots of tentacles, surrealism, trippy, go for it. Flying castles on the backs of sky whales lording over the land while purple clouds hang in the sky dropping red rain. Why in heck not?
Hey, with the recent release of John Carter, it's not too hard to get "weird" in the art.