I think Zoro should have fought Shamrock (especially since he has the Cerberus sword—3 dog heads against the 3-sword style) + Shamrock poses at least some threat, unlike Sommers and Killingham.
Sanji could have fought Sommers, because at the beginning of the arc he talked about love, and that would be a perfect opportunity to explore the theme of Sanji's "power of love." Plus, he could counter Sommers' spikes with his fire.
And Killingham should have gone to Usopp. It would have been the perfect fight for him. Throughout the battle, Killingham could have brought out all of Usopp's fears, and Usopp would gradually overcome them, finally becoming a brave warrior of the sea (maybe even awakening Conqueror's Haki). This would be the best upgrade for Usopp, and right in the Elbaf arc.