The loose end in question is the Council of Kangs, established as the primary villains of the Multiverse Saga in Ant-Man and the Wasp: Quantomania’s end-credit scene. At the end of the film, a collection of the strongest Kang the Conqueror variants gathered to address the threat the Avengers pose to their plan. This seemingly set the stage for what was supposed to be Avengers: The Kang Dynasty, though Marvel has obviously had cause to change their plans of late.
Pivoting the events of Avengers 5 toward Doom makes sense, especially given the set-up of Avengers: Secret Wars’ story. However, it would be unwise of Marvel to do so without addressing what happened to the Council of Kangs. As such, I think the best way to open Avengers: Doomsday is to put audiences in the point-of-view of RDJ’s Doctor Doom, establishing his backstory and his plan to dominate the multiverse. This should end – ironically at the beginning of Avengers: Doomsday – with Doom decimating the Council of Kangs, removing the prior Multiverse Saga villain once and for all.
Thank You So Much For Watching!