We don't need another expansion that milks our nostalgic udders any harder. They had two of those, in a row.
We need another Mists of Pandaria. We need another expansion that expands on the lore, that sets a fresh eye to the faction conflict, that tightens gameplay and brings new ideas to the table.
In other words, we need all the things BfA tried to do, but not done horribly.