This Arc started with Vivi leaving a message for SHs on Newspaper, so its obvious that something in Elbaph will remind them of her, and convince them to go look for her, in whjch case Robin would tell them about X Mark, that's the whole reason Arabasta SHs were separated at first.
What's in the Chest is something that Vivi needs, and as I mentioned before, whenever there is a Special Army, Oda will introduce a Female who can command them, such as Shirahoshi with Sea Kings, Perona with certain Zombies, Tama with Smile Army, Nami with Homies, Bonney with Pacifista ... etc
Imu's Army needs a Girl who can do that and its definitely Vivi, so what's in the chest would allow her to have such Ability,
The chest contains counter to corruption (Domi Reversi), covenants and omen, but Vivi isnt there, Oda needs someone else to temporarily test that power, someone who shares a physical feature with Vivi and that's Gunko and her blue hair,
I feel like Gunko, Sommers and Killingham will be freed from Imu, thats why these sub-plots are happening at same time,
Imu will be forced to go look for vivi himself asap, which is the main trigger of next arc.
