Velen - a 100 000 yo priest who lost his people, lost his son, lost his home and struggles to get his revenge against those who betrayed him.
Trall - a young orc slave who ended up building a New Horde and representing Earthern Ring.
Saurfang - not really well deveoped except for losing his son during ICC events.
Vol'Jin - is vastly underveloped. Has so much potential. Still helped in recreating the Horde, lead the rebellion against Garrosh'es tyranny.
Boy King - has much potential. Lost his father at young age. Burden of being a King of Alliance. Needs to sort out what is right and what is wrong for himself. Right after he becomes a King, we get another faction war. And we know he has plot armor till he is like 70 yo badass leader of the fight against the Void. So he has much room for development.