Realistically, any of the champs *could* move up a weight class and have a shot at winning the title against the current champ. Becoming a champ in the UFC in the last 15 years shows someone is an elite MMA fighter so they will always have a chance against someone who is only a weight class bigger than they are. Most would be pretty substantial underdogs though.
For instance Volk certainly could lamp Do Bronx and Do Bronx could grab a heel hook or other leg attack against Usman. May not likely, but certainly possible. Maybe the one outlier is Jan vs Ngounnu. Not sure that Jan has a path to victory there but the size difference would be immense and there are no other 2 weight classes that are anywhere as far apart as LHW and HW.
To your point about Silva, at no point would I have favored him against Jones or DC though he'd at least have a fighting chance against DC. Prime Jones beats prime Silva 98 out of 100 times IMO.
Aldo certainly could have beat Pettis. At that point he probably would have been favored to win.