In regard to bringing The Grandmaster back, a great idea would be to have him seak out Beta Ray Bill to try and get revenge on Thor, Valkyrie, Korg, Miek, the gladiators that escaped... I mean they took his champion/Hulk away, killed Topaz his right hand woman, stole his property, started a revolution, destroyed parts of Sakaar, almost toppled the dictatorship...
Grandmaster has plenty of motives to get back at Thor, he has the capability, the resources... And he's the one that has the strongest connection with Beta Ray Bill since the korbinite was one of his champions. Maybe the Grandmaster tricks BRB into thinking they are the enemy and puts them in a collision course.
It's a great way of bringing not only the Grandmaster but BRB into the fold. It all lines up perfectly.