He has the potential, that's for sure. But "he can only get better" doesn't work in reality. Just look at Alexis or Cesc. Pedro, Busquets or Pique even. All looked much better in their first seasons.
I don't really want to split hairs, but:
Alexis looked 'better' the first couple of games, than he was mediocre until about 10 games ago when he really started to impress on me.
Cesc looked lost the first season, than was brilliant the fist half of this season - than dipped again.
Busquets had his real first drop in form second half of this season.
Pedro and Pique - agree.
Anyhow, back to ter Stegen. He looks like the real deal, definitively one of the most impressive out of the younger generation. I say why not?!