Gunko, Sommers and Killingham are already defeated once, Oda doesn't like to repeat himself so I don't think we're getting main fights with these three, at the least not M3 fights.
A weak or mid trio fight though, there is a good possibility there. I could see Robin or Nami vs Sommers. The guy likes to hurt women and children, so he's a good punching vag for one of the two lady strawhats.
I could also see Usopp vs Killingham. It's Usopp vs Perona 2.0. Usopp overcoming his fears/nightmares as a brave warrior of the sea is a fertile ground for great Usopp moments.
Gunko however is the arc princess so she's automatically out as a possible antagonist.
Sanji and Zoro will get their opponents ofc, and I think they're going to be either one of the Gorosei each, or the as if now not introduced right and left hand of Shamrock amongst the HK.
Shamrock himself is Loki's final opponent imo.