The top contender has to be Fawad Alam in my opinion. I really think he will do well in test cricket. I know our team has an attritional batting style, but test cricket is the one place where this can bring success, and it has brought us success in the recent past. Fawad is the perfect replacement for Misbah.
Younis Khan on the other hand has no replacement. He's a once-in-a-generation talent from Pakistan. As far as world-class batsmen who average 50+, we had Miandad way back when, and then the Inzi/Yusuf era, and the lone YK era. No one in the Pakistani setup seems to have the mettle to average 50+ yet, though Azhar is getting there slowly. Babar Azam is someone who we are placing our trust in, but it is way too early to say. YK's loss will be huge for Pakistan!