It might have changed a lot. For starters, they would have had the support of the Alliance in dealing with the Scarlet Crusade - which is a big deal. Second, the Alliance would still have set up in Lordaeron. And considering the Blood Elves connected with the Forsaken on the back of their shared disconnect from others and Sylvanas, perhaps it would have been the Blood Elves that joined the Alliance too. Also, that by extension means the Nightborne would have joined the Alliance as well. And without Nathanos rescuing Talanji, the Zandalari may never have accepted the Horde.