Maybe new blood, but that's not enough.
We need designers that are humble enough to go back to the roots and understand why things were designed the way they were, and how not understanding this led to worse and worse decisions over the years: if a game is well done you can't simply decide to wipe this and that talent and switch this from a class to another or from a spec to another, because balancing is very hard, interesting gameplays and depth of play are very precious.
Designers that understand a new expansion is more than a single gimmick: let's replace talent trees with a few rows of 3 fake choices, let's give every capability to every class,let's remove 90 % of stats, and let's invent a few new retarded ones, because we're more than an RPG, let's steal most capabilities from most classes (pruning), let's force people to play a single spec and let's arbitrarily attribute capabilities to one spec only (class fantasy).
Designers who understand you can't be everything to every gamer, and understand there is no future in a 3 - button MMORPG, that the 24/7 grinding is already covered by Korean MMORPGs.
Converserly designers who play other games and are willing to take greats features from a game if its adaptable and relevant to WoW.
Designers who are ready to listen to players when possible (we want housing) and not systematically overdoing it to the extreme (garrisons).
And of course, to recognize the very few good ideas, such as reforging, that were added to the game over the years, and cherish and protect them.