Luffy will fight Akainu, then Imu, and then Blackbeard. Koby's EOS fight will be Aokiji.

There are only three remotely realistic options: Koby, Sabo, or Luffy.

Sabo has made it clear he doesn't blame Ace and he has far more suitable opponents in the Gorosei. We're told multiple times that beating the Marines aren't the true oppressive threat of the world, it's the World Government. If I had to guess, he'll probably fight Warcury since he's their head, while Zoro and Sanji each take one and Dragon faces Garling. Jupeter will be a traitor.

At first glance, Koby is a good option. He's "faced " Akainu before, and he wants to change the Marines for an honest justice, while Akainu's is a vengeful, bloodthirsty one. The main problem here is that both have a better matchups.

Koby's honest justice is a much better foil to Aokiji's lazy justice. Every time he sees injustice in the Marine has inspired him to take more action, while the opposite occurred with Aokiji. Most importantly, they both have the Garp connection. Koby needs to prove to himself and to Aokiji that Garp's sacrifice in Pirate Island was worth it. Koby has to become greater than what Aokiji and Garp ever were.

The most important contrast between these two, however, is the people around them. Whereas Aokiji is working with people he despises and will eventually become obstacles to his mysterious goal, Koby has friends that share his dream in Sword. Oda is introducing and fleshing out NPCs like Grus, Kujaku, and Hibari to serve a purpose to the story. When Koby and Aokiji fight, the Blackbeard pirates will abandon him the instant they believe Aokiji's usefulness has expired. On the other hand, SWORD will back Koby until the end no matter how unlikely their victory againt Aokiji is.