He has a quality left foot but it is limited to ball-striking and crossing. He's not an outstanding passer in terms of through balls, or pass and move. He's decent in that regard but I think compared to your Ozil's and David Silva's he's not quite got their finesse or off the ball movement.
Dribbling wise, again he's not really a top tier talent with high level of nimbleness or vast array of tricks.
Athletically again he lacks top level pace and acceleration, but I'd say he's solid enough in that regard to be more explosive than a Januzaj (he's definitely better than him IMO) but he's never quite going to be a Bale and run defenders ragged.
So in terms of glass ceiling, I can't really see how he's going to end up as either an era defining wide player or an era defining attacking midfielder. But he does have a solid all-round game, he's not below par physically, or in terms of technical skills, or in terms of football intelligence and he has a good attitude. It's just outside of his shooting, he lacks 'wow factor' in any other aspect of his game.
So I can see him being a good servant for Madrid over the years but not as their 'driving force'. He is a support act, not the main star of a side and certainly not as the star of a team the size of Real Madrid.