It's interesting to speculate about if Zaarin had succeeded in his coup attempt against Palpatine. The Grand Admiral came very close, ambushing Vader, incapacitating and capturing the Emperor, then even when the Emperor was rescued by the combined efforts of Thrawn, Vader and Ace Pilot Maarek Stele, Zaarin took his sizable fleet with its advanced TIE Defenders to lay waste to the Empire's Advanced Starfighter production facilities.

If just a few things had gone slightly differently, such as Maarek Stele not being reassigned to Vader's task force for the battle of Ottega, or Zaarin choosing to just destroy the Emperor's ship rather than capture the Emperor first, Zaarin would have effectively decapitated the Empire six months before the battle of Endor-likely ushering in the chaos of the Warlordism that tore the Empire apart after Endor in the original timeline. 

Just as interesting to speculate upon is if Zaarin had waited six more months to launch his bid for the Imperial Throne, Palpatine and Vader would have been dead, with Thrawn in the Unknown Regions. Zaarin would have been poised to become the dominant Imperial Warlord with his focus on Starfighter as well as technological Superiority, flexibility and practical fleet composition.

Grand Admiral Demetrius Zaarin. Inventor of the TIE Avenger, Defender, and TIE Experimentals. He led a coup against Palpatine in 3.5 ABY nearly killing the Emperor and Vader. His Insurrection crippled starfighter production in the Empire before he was finally defeated by Thrawn.