Well even if he was still there I doubt he'd work on D4 anyway, his name is tainted for the Diablo franchise for sure. I think if they used the current Diablo guys they'd probably make a pretty good game. I kinda wish they had more depth to builds and characters and shit like that, as stuff is relatively cookie cutter in the current game. Such as Sets > everything besides a few specific builds with LoN but since they forgot to buff LoN and buffed almost every set this patch that's also kinda obsolete, and once you have 6pc of a set, plus the 2-3pieces that make the set actually viable designed entirely to work with the set, that leaves you with very few choices, and mostly illusion of choice when some of the "choices" like F&R or Traveller are so stupidly strong. So classes have like 4-5builds each or so, but not really ways to deviate from these or customize them very much, which is a bit boring. Which isn't bad, considering the amount of classes, it just doesn't open much room for discovery when everything is dictated by the presence of sets.
Also stems heavily from runes not being equal at all, so if set a buff skill x, you'll probably always pick rune y on skill x because it's the best rune, and you'll build the rest of your stuff to match rune y element, forcing you into specific runes for everything else too, to an extent. There's some skills that see interesting choices like Multishot with cold versus fire, but so many skills are just 1 viable rune and then maybe one support one and one leveling one, but mostly a lot of useless ones like increased slows and chance to stun and shit which with the way CC is treated are close to useless.
Anyway still, game would be a ton more fun than D3 at release for sure. Although I think I'd still rather get an expansion than a new Diablo. If they make a new game they'd have to restart from nothing, which means it'd be lacking a bunch of features. I'd rather new features be added to current feature rich D3. Any news at all would be good though, fucking Blizzard.