I actually do think his shot selection has been much better this year than last year, not to the point where he can construct points effectively but at least to the point where he doesn't open up 95% of the court by hitting crosscourt or down the line when he really should be doing the other.
On surfaces other than clay (which I doubt he'll ever be great on) it's obvious that the main improvements he can make are:
*His return game (big backswings limit him quite a bit but a solid chip on first serves like Wawrinka has could do wonders for him.
*Maybe a better grasp of when to slice and when to attack with the backhand. I do believe his backhand is flawed in general though especially when it comes to controlling direction on defensive/grinding shots, or even when controlling spin and power.
On clay he could also do with better decision making on the backhand side, and patience on the forehand side especially when things aren't going his way.
Another thing I noticed today was he often couldn't hit his backhand deep enough or with enough pace when under pressure from Nadal's high-bouncing forehand, and Nadal being Nadal was able to pounce on them quickly. He also, as mentioned, just can't control where that thing is going when time is taken away from him.
Also, Gunter Bresnik is so power obsessed from what I've heard it's ridiculous, Thiem really just needs to learn how to construct points to suit the weapons he has. I sometimes wonder if he would have any success if he started slicing his backhand even more and only went for topspin when there's a clear opportunity to seal a point, modelling his game completely around controlled aggression with the forehand. That's probably an outrageous idea but despite most unforced errors coming off the forehand today I really don't trust his backhand to work consistently.
When I think about it his game is technically flawed just as much as it is tactically flawed due to his swings. I've accepted he probably won't be the next big thing but might win a masters or even a slam here and there in the future.
And who knows, maybe he'll just keep working on mastering his own game and become the greatest, most powerful, most exciting ball-basher in history. I certainly would love to see him smacking 100MPH forehand winners into the corners every other point.