I don’t really see Wheeler coming back, so we are likely looking at Toppin starting at the 4.Was supposed to have happened this season too and then there was Oscar, Toppin and Brooks or a combination of 2-3 bigs on the court most all of the time. No way Toppin doesn't start next season and he's not playing the 5.
But in the scenario you listed, I think Wallace, Cook, and Livingston will start, based on Cal’s track record. He doesn’t bring in talent like those three and not play/start them. They are guards/wings, so that would leave two spots and one is the 5 spot. So unless Toppin plays the 5, or Wheeler goes to the bench, Toppin likely remains a reserve. Maybe it’s Wallace, Cook, Livingston, Toppin, and a Big (Collins/Transfer) with Wheeler being the spark plug off the bench. Either way, Wheeler and Toppin are in the rotation.