I have a couple theories about some of halo wars 2’s characters .
First of , I think that the chieftain from the first halo wars is probably Atriox SInce we know that the spirit of fire as been drifting since around 20 years ,This would have given him the time to leave the covenant and form the banished . Plus , we know that they are both very smart and are pretty old .
My second theory is that Let Vohir , the elite fleetmaster from halo wars 2 will eventually side with the UNSC and help them fight the banished . Now , this is just speculation , but it would make sense because we know that he joined Atriox because that was the only way to get fuel and resources for his fleet . Plus , elites and brutes tend to dislike each other and maybe he will learn about the swords of shangheillios somehow and want to join them.