A world class striker won't turn this team from good to excellent. That depends on how we work out defensively. Even though we might not sign any defensive players, I think with properly defined roles, we can make a lot of progress.
Last season no midfielder seemed to know what they were doing. It was choas. If we tell Arteta to play purely as a DM in the Busquets role, and we tell Song he no longer has to assume as much attacking responsibility, our shape will be a lot better.