#1 Not making him the Universal Champion
There are many reasons why The Fiend should be the Universal Champion by the end of 2019. One, the momentum he has behind him is absolutely incredible and WWE should hit the iron when it is hot.
Second, there isn't a more deserving Superstar than Wyatt that deserves to be made the Universal Champion. Another reason is that WWE needs a top heel champion as Seth Rollins has held on to his title for long enough.
The Fiend should be the Superstar to beat Rollins for the Universal Championship and this could happen as soon as Hell in a Cell. The Fiend as the Universal Champion is going to drive the WWE Universe nuts. Even though Wyatt is a heel, he would get a huge pop if and when he shows up with the red title hanging around his waist.