luffy doesn't have any reasonable motivation to fight blackbeard to be honest.
It was ace who started the fight with blackbeard, and blackbeard beat ace fairly.
Same can be said for akainu. Akainu just did his job as a marine. Again, it was ace who started to attack akainu, only to get beaten hard.
Imu is near end character, no rivalry hype
Meanwhile, zoro vs mihawk has been hyped for a long time. A fight foreshadowed since the early part of this series