First step is to remove auto tackling. Why is there a tackle button if you never need to press it? It seems simple, but if you don't press the tackle button you shouldn't get a tackle. Easy.
Next is to reduce the effectiveness of slide tackles. The types of angles and distances covered by slides is stupid in this game. They shouldn't be so accurate, especially by attacking players.
Make the tackling ratings mean something. If a player controls his striker to track back and make tackles, (I'm looking at you Aubamayeng) then he should be committing more fouls and having a lot less success with his tackles. As it stands there is virtually no difference between 90 standing tackle and 50 standing tackle.
Also get rid of that thing where I make a perfect tackle or interception and the ball just bounces right back to an attacker. This can also be coded into the tackle rating. The higher the rating, the higher the chance they have to keep possession when making a tackle. I also think this has some to do with auto tackling. The attacking player being so close to the defender just triggers and auto tackle animation giving him the ball right back.
If I make an interception, or my opponent passes it directly to my defender, don't punish my good play by having the defender take an awful touch and the ball just bounces right into the path of the attacker. That's one of the most irritating things in the game.
I've come across so many players that, when they see my attacker running towards them in space, they pull one of their CB's out of position sprinting like a mad man. That should never work. That's an awful way to defend and it shouldn't be effective. Hopefully the improved runs by your AI teammates will help with exploiting those gaps a lot better.
These are the things I want to see improved on the defensive side for FIFA 17. I think all of these things are really easy to implement and I can't wait to see absolutely none of this change. Lol