I got a few examples on this one.
#1 Eddie Guerrero/Kurt Angle-now I know they faced each other at a Wrestlemania, and at Summerslam, but, still, with these two, they should have main evented one of the two shows, and neither one happened. If Angle didnt get hurt in their Wrestlemania match, and have to take the GM role for a few months, maybe it could have had a better ending.
#2 Rey Mysterio/Chavo Guerrero-it shocked me that they put this match on a Summerslam, but, still, very underrated feud. They could have had this one go on for multiple PPVs, but, just dropped it after Summerslam. They should have built up Mysterio's injury off of it, and had it last longer than a month in my opinion.
#3 Chris Benoit/MVP-this PPV lasted for 3 months, but, still, in the end, in a 2 out of 3 falls match, MVP won it, and Benoit did next to nothing in the match, losing his US title in the process. This was Benoit's last feud before the events that changed pro wrestling forever. Just seemed like whenever they faced each other, no one cared, and I think with better booking, this feud could have been so much better.
Finally #4 Taz/Kurt Angle-talk about a feud that should have lasted more than one month, if it was even considered a feud. After Tazz beat Kurt Angle at the Royal Rumble in his debut, soon after that, he was buried. This feud could have stolen the show for the "true" wrestling fans, but, they decided to end it, didnt make much sense to me, and could have been an awesome feud.