Usopp, Zoro, Kaku, and Killingham all share associations with giraffes.
In JP, "giraffe" & "kirin" are interchangeably used, as キリン refers to both. For example, Kaku was referred to as Kirin Ningen in the JP RAW and Giraffe Man in VIZ's translation. Since Zoro has already defeated Kaku, it makes sense for him to also defeat Killingham, the mythical Zoan-user counterpart.
Zoro's first meeting with Kaku was played as a gag where he mistook Kaku for Usopp. Later, Zoro wielded Usopp as a blade to temporarily fight Kaku (and Jabra). I don't think Usopp will fight alongside Zoro against Killingham, but I do believe he will steal Killingham's horn, which is likely inspired by the Norse mythological Gjallarhorn. With the horn, "God" Usopp could command an army of MMAS or Sleeptids against Imu and the Holy Knights.
I also believe the Huntsmen Village of Elbaph could offer a potential path of growth for Usopp. He has been associated with archery through various cover pages and color spreads.