Say Doomhammer had killed Gul'dan and Cho'gall after they created the first death knights. They wouldn't have taken troops away from the siege of Lordaeron and Orgrim wouldn't have diverted his forces which would allow him to crush Lordaeron effectively winning the Second War. I think one of three things would happen afterwards:
Number One: They would have methodically gone from kingdom to kingdom (Stromgarde, Gilneas, Ironforge, Quel'Thalas, etc.) and destroyed all opposition. After conquering all of the Eastern Kingdoms they would have taken all of the viable resources from Draenor and had all the clans that remained their (Bonechewer, Shadowmoon, Mag'har, etc.) move to Azeroth. Thrall would be raised by his family since Gul'dan never had assassins to kill Draka and Durotan. Mannoroth would come to Azeroth to assert his dominance over the orcs, but Grom Hellscream would slay him and die (like in the original timeline) and Garrosh would take his position since he learned orcish traditions being raised in a proper orc society. After the release of the Blood Curse, Warchief Doomhammer realizing the harmful effects of demon magic, slaughters the remaining death knights and bans fel magic. Thrall, trained by Drek'thar, would show much promise and is taken in as apprentice to the former Warchief, Ner'zhul. With Ner'zhul's guidance Thrall would be able to reestablish his people's connection to the elements and the ancestors. With Ner'zhul's death Thrall would become the advisor to the aging Warchief Doomhammer. After his eventual death Thrall is given the title and becomes the new Warchief bringing a new age of prosperity to his people.
Number Two: They conquer all the kingdoms. Mannoroth returns and is able to make the orcs submit. He sends the orcs across the ocean to slaughter the night elves. After they destroy most of the night elf population and burn down Ashenvale/Darkshore the legion twists them completely into fel orcs and adopts them into their army to continue their conquest of the worlds.
Number Three: They capture Lordaeron, but are defeated after all the remaining kingdoms unite for one final stand at Stromgarde. Without the voice of Terenas to persuade them to force the orcs into internment camps, all the orcs are executed. The united kingdoms march into the Dark Portal and finish off the remaining clans who are taken by surprise. They then take any resources Draenor has to offer and close the portal behind them when they are finished.
Thoughts? Please post anything you also think would have happened.