Even if I wanted it, I was a bit skeptical that all the Generation III Pokemon would receive an evolution at first considering that they're not TOO different from Sugimori's current style. And with Groudon and Kyogre featuring their old artwork, I'm kind of thinking that we won't have new Sugimori art for every single Gen III Pokemon, at least. I mean even if they had new mega evolutions, why wouldn't Sugimori draw new artwork of regular Kyogre and Groudon? I do think that the Starter Pokemon will get new artwork though, if only because they always seem to get new artwork for third versions/sequels/remakes.
Just compare this:
and that isn't as drastic as
So while I'd wish for re-Sugimori of every Hoenn Pokemon, I don't think we'll be getting it. The Gym Leaders, Elite, and other important guys are definitely getting new artwork though. If only because they're probably all going to be redesigned. And like @The Guardian Sylveon said, the human designs are really different now, especially the eyes.