Here's the thing. Blackbeard is the ONLY person to have eaten more than one DF.
That makes him special. Akainu really isn't special. He barely beat out his fellow admiral after 10 days.
You simply can't compare the importance of Blackbeard who is a D with Akainu.
More things that are crazy about BB will be revealed.
Every single "final" character is special in their own way lol, thats precisely why I think all of this is going to go down very close or near the ending anyway.
- Blackbeard being a D. doesnt necessarily mean he will "outlast" other characters, same with his unique body (I assume he and his crew eat the heart of the dead person they steal powers from). I DO agree he has endgame relevancy
- Akainu killed Ace and scarred Luffy.... he by far has had more negative impact on the MC than anyone else
- Shanks, being the most enigmatic of the 3 so far, he is the main drive of the MCs dream
- Imu is essentially the last unrevealed villain the series (unless Elbaf has something else and the Burned Man is a villain) and seems to be the natural direct enemy of Luffy.
All in all, all 4 have viability to be endgame threats, though I argue Akainu less so because he doesnt seem to be a CD nor D. Clan member. I get this, but what Im suggesting has to do with the fact that Oda highlighted both similarly pre and post TS.
Shanks and Imu do not have this, and almost seem to fit into a completely different category on relevancy.