I have following scenarios:
One: - Imu comes here/boost Kilingham and Sommers and really wrecks anything, drops some lore drops and leaves because of a timelimit
(In this scenario the arc would last at best 10 Chapters, since Imu coming and wrecking around would take 3-5 Chapters at best).
Two: - Imu comes, Alliance fights back, Sommers and Kilingham getting 1on1 battles, the Alliance don´t get defeated
(In this scenario the arc would last over 20 chapters).
Third - Imu comes>little break to see a overview situation, third fraction joins the party
(in this situation the end of this arc could take over 30 chapters).
My guess given all the tease/hints, scenario one will most likely happening.
Imu wrecking everything and leaving...