Barkley has wonderful natural gifts very few other players possess, namely the pace, strength, and movement he displays with the ball at his feet. There are only a handful of players across Europe capable at carrying the ball over distance like he can.
That's a talent that should be celebrated, but it doesn't mask his limitations: some rudimentary aspects of his game haven't been developed yet, including his positional sense and decision making. There's also more difficult skills that need to be improved if he's going to be the player we hope, such as the quality of his final ball.
That's where he's at right now: a 20 year-old with extremely rare talents, but one who's football education is far from over - especially considering a few injuries that have hampered his development. If he stays clear of serious injury and has two full seasons of regular starts we will hopefully see an outstanding all-round footballer, which is why any move to City/Chelsea in the short term would be a big mistake. He's at the ideal club to fulfil his potential.