#2 Lesnar attacking the Shield
One of the reasons why this feud doesn't seem interesting is due to the fact that both Lesnar and Rollins weren't involved much in this. Of course, they had few faceoffs and Lesnar even attacked Rollins but still, that didn't do much.
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In the meantime, Rollins was also involved in the Shield reunion at the road to WrestleMania. It was exciting to have a mini-Shield reunion but it only hurt Rollins' major storyline. It took his concentration totally away from Brock Lesnar and the WrestleMania match. It would've been better if Lesnar got involved in Shield's reunion to some capacity.
Lesnar ruining the Shield reunion would've given Rollins a solid motivation to defeat him. Not using Lesnar in the angle is a huge missed opportunity. It would've created an awesome WWE moment if Lesnar attacked the Shield after their Fastlane match. It would've also made Rollins fight for pride in addition to the universal title.